ox-icalendar: Avoid processing ignored elements

* lisp/ox-icalendar.el (icalendar): Use nil transcoders instead of
`ignore' for ignored element types.  Otherwise, `org-export-data'
still exports the ignored element contents, including resolving links,
and the result is ignored.  The contents might contain broken links
and hence lead to errors despite the whole element not supposed to be
exported.

Fixes https://orgmode.org/list/45aa417a-7eb1-1070-b0dd-17bdd7d1448f@mailbox.org

lisp/ox-icalendar.el

@@ -276,14 +276,14 @@ re-read the iCalendar file.")
 ;;; Define Back-End
 
 (org-export-define-derived-backend 'icalendar 'ascii
-  :translate-alist '((clock . ignore)
-		     (footnote-definition . ignore)
-		     (footnote-reference . ignore)
+  :translate-alist '((clock . nil)
+		     (footnote-definition . nil)
+		     (footnote-reference . nil)
 		     (headline . org-icalendar-entry)
                      (inner-template . org-icalendar-inner-template)
-		     (inlinetask . ignore)
-		     (planning . ignore)
-		     (section . ignore)
+		     (inlinetask . nil)
+		     (planning . nil)
+		     (section . nil)
 		     (template . org-icalendar-template))
   :options-alist
   '((:exclude-tags