org-clock: Fix `org-clock-remove-empty-clock-drawer'

* lisp/org-clock.el (org-clock-drawer-name): New function.
(org-clock-find-position): Use new function.
(org-clock-remove-empty-clock-drawer): Remove drawer actually used for
clocks, which may be different from the one used for storing notes.
Also, consider that headlines could use different drawer names in the
same tree.

lisp/org-clock.el

@@ -556,6 +556,13 @@ of a different task.")
   (mapc (lambda (m) (org-check-and-save-marker m beg end))
 	org-clock-history))
 
+(defun org-clock-drawer-name ()
+  "Return clock drawer's name for current entry, or nil."
+  (let ((drawer (org-clock-into-drawer)))
+    (cond ((integerp drawer) (org-log-into-drawer))
+	  ((stringp drawer) drawer)
+	  (t nil))))
+
 (defun org-clocking-buffer ()
   "Return the clocking buffer if we are currently clocking a task or nil."
   (marker-buffer org-clock-marker))
@@ -1451,10 +1458,7 @@ line and position cursor in that line."
     (let* ((beg (line-beginning-position 2))
 	   (end (save-excursion (outline-next-heading) (point)))
 	   (org-clock-into-drawer (org-clock-into-drawer))
-	   (drawer (cond
-		    ((not org-clock-into-drawer) nil)
-		    ((stringp org-clock-into-drawer) org-clock-into-drawer)
-		    (t "LOGBOOK"))))
+	   (drawer (org-clock-drawer-name)))
       ;; Look for a running clock if FIND-UNCLOSED in non-nil.
       (when find-unclosed
 	(let ((open-clock-re
@@ -1646,18 +1650,19 @@ to, overriding the existing value of `org-clock-out-switch-to-state'."
 
 (add-hook 'org-clock-out-hook 'org-clock-remove-empty-clock-drawer)
 
-(defun org-clock-remove-empty-clock-drawer nil
-  "Remove empty clock drawer in the current subtree."
-  (let ((clock-drawer (org-log-into-drawer))
-	(end (save-excursion (org-end-of-subtree t t))))
-    (when clock-drawer
-      (save-excursion
-	(org-back-to-heading t)
-	(while (and (< (point) end)
-		    (search-forward clock-drawer end t))
-	  (goto-char (match-beginning 0))
-	  (org-remove-empty-drawer-at (point))
-	  (forward-line 1))))))
+(defun org-clock-remove-empty-clock-drawer ()
+  "Remove empty clock drawers in current subtree."
+  (save-excursion
+    (org-back-to-heading t)
+    (org-map-tree
+     (lambda ()
+       (let ((drawer (org-clock-drawer-name))
+	     (case-fold-search t))
+	 (when drawer
+	   (let ((re (format "^[ \t]*:%s:[ \t]*$" (regexp-quote drawer)))
+		 (end (save-excursion (outline-next-heading))))
+	     (while (re-search-forward re end t)
+	       (org-remove-empty-drawer-at (point))))))))))
 
 (defun org-clock-timestamps-up (&optional n)
   "Increase CLOCK timestamps at cursor.