ox: Remove `org-export-get-headline-id'

* lisp/ox.el (org-export-get-headline-id):
(org-export--collect-unnumbered-headline-id): Remove function.
(org-export-collect-tree-properties): Apply removal.

* testing/lisp/test-ox.el (test-org-export/org-export-get-headline-id):
  Remove test.
(test-org-export/numbered-headline-p): Update test.

`org-export-get-reference' supersedes it.

lisp/ox.el

@@ -1710,7 +1710,6 @@ Return updated plist."
   ;; properties.
   (nconc
    `(:headline-numbering ,(org-export--collect-headline-numbering data info)
-     :unnumbered-headline-id ,(org-export--collect-unnumbered-headline-id data info)
      :exported-data ,(make-hash-table :test 'eq :size 4001))
    info))
 
@@ -1757,17 +1756,6 @@ for a footnotes section."
 		   when (> idx relative-level) do (aset numbering idx 0))))))
       options)))
 
-(defun org-export--collect-unnumbered-headline-id (data options)
-  "Return numbering of all exportable, unnumbered headlines.
-DATA is the parse tree.  OPTIONS is the plist holding export
-options.  Unnumbered headlines are numbered as a function of
-occurrence."
-  (let ((num 0))
-    (org-element-map data 'headline
-	(lambda (headline)
-	  (unless (org-export-numbered-headline-p headline options)
-	    (list headline (incf num)))))))
-
 (defun org-export--selected-trees (data info)
   "List headlines and inlinetasks with a select tag in their tree.
 DATA is parsed data as returned by `org-element-parse-buffer'.
@@ -3721,9 +3709,6 @@ process, leading to a different order when footnotes are nested."
 ;; `org-export-get-headline-number' returns a number to unnumbered
 ;; headlines (used for internal id).
 ;;
-;; `org-export-get-headline-id' returns the unique internal id of a
-;; headline.
-;;
 ;; `org-export-low-level-p', `org-export-first-sibling-p' and
 ;; `org-export-last-sibling-p' are three useful predicates when it
 ;; comes to fulfill the `:headline-levels' property.
@@ -3757,17 +3742,6 @@ and the last level being considered as high enough, or nil."
       (let ((level (org-export-get-relative-level headline info)))
         (and (> level limit) (- level limit))))))
 
-(defun org-export-get-headline-id (headline info)
-  "Return a unique ID for HEADLINE.
-INFO is a plist holding contextual information."
-  (let ((numbered (org-export-numbered-headline-p headline info)))
-    (concat
-     (if numbered "sec-" "unnumbered-")
-     (mapconcat #'number-to-string
-		(if numbered
-		    (org-export-get-headline-number headline info)
-		  (cdr (assq headline (plist-get info :unnumbered-headline-id)))) "-"))))
-
 (defun org-export-get-headline-number (headline info)
   "Return numbered HEADLINE numbering as a list of numbers.
 INFO is a plist holding contextual information."

testing/lisp/test-ox.el

@@ -1946,53 +1946,19 @@ Paragraph[fn:1]"
    (org-test-with-parsed-data "* Headline"
      (org-element-map tree 'headline
        (lambda (h) (org-export-numbered-headline-p h info))
-       (plist-put info :section-numbers t)))))
-
-(ert-deftest test-org-export/org-export-get-headline-id ()
-  "Test `org-export-get-headline-id' specifications."
-  ;; Numbered headlines have IDs akin to "sec-N".
-  (should
-   (equal "sec-1"
-	  (org-test-with-parsed-data "* H"
-	    (org-export-get-headline-id
-	     (org-element-map tree 'headline #'identity info t)
-	     info))))
-  ;; The ID of numbered headlines reflect the hierarchy.
-  (should
-   (equal "sec-1-1"
-	  (org-test-with-parsed-data "* H1\n** H2"
-	    (org-export-get-headline-id
-	     (org-element-map tree 'headline
-	       (lambda (h)
-		 (and (equal "H2" (org-element-property :raw-value h)) h))
-	       info t)
-	     info))))
-  ;; Unnumbered headlines have IDs akin to "unnumbered-N".
-  (should
-   (equal "unnumbered-1"
-	  (org-test-with-parsed-data
-	      "* H\n:PROPERTIES:\n:UNNUMBERED: t\n:END:"
-	    (org-export-get-headline-id
-	     (org-element-map tree 'headline #'identity info t)
-	     info))))
-  ;; The ID of Unnumbered headlines do not reflect the hierarchy.
-  (should
-   (equal "unnumbered-2"
-	  (org-test-with-parsed-data
-	      "* H1\n:PROPERTIES:\n:UNNUMBERED: t\n:END:\n** H2"
-	    (org-export-get-headline-id
-	     (org-element-map tree 'headline
-	       (lambda (h)
-		 (and (equal "H2" (org-element-property :raw-value h)) h))
-	       info t)
-	     info))))
-  ;; When #+OPTIONS: num:nil all headlines are unnumbered.
-  (should
-   (equal "unnumbered-1"
-	  (org-test-with-parsed-data "* H\n#+OPTIONS: num:nil"
-	    (org-export-get-headline-id
-	     (org-element-map tree 'headline 'identity info t)
-	     info))))
+       (plist-put info :section-numbers t))))
+  ;; With #+OPTIONS: num:nil all headlines are unnumbered.
+  (should-not
+   (org-test-with-parsed-data "* H\n#+OPTIONS: num:nil"
+     (org-export-numbered-headline-p
+      (org-element-map tree 'headline 'identity info t)
+      info)))
+  ;; Headlines with a non-nil UNNUMBERED property are not numbered.
+  (should-not
+   (org-test-with-parsed-data "* H\n:PROPERTIES:\n:UNNUMBERED: t\n:END:"
+     (org-export-numbered-headline-p
+      (org-element-map tree 'headline #'identity info t)
+      info)))
   ;; UNNUMBERED ignores inheritance.  Any non-nil value among
   ;; ancestors disables numbering.
   (should