org.el: Fix `org-comment-string' fontification

* org.el (org-set-font-lock-defaults): Don't fontify
`org-comment-string' when not in a headline.

lisp/org.el

@@ -6365,7 +6365,6 @@ needs to be inserted at a specific position in the font-lock sequence.")
 			     ":\\).*$")
 		     '(1 'org-tag-group prepend)))
 	   ;; Special keywords
-	   (list (concat "\\<" org-comment-string) '(0 'org-special-keyword t))
 	   (list (concat "\\<" org-deadline-string) '(0 'org-special-keyword t))
 	   (list (concat "\\<" org-scheduled-string) '(0 'org-special-keyword t))
 	   (list (concat "\\<" org-closed-string) '(0 'org-special-keyword t))