ox: Remove filter associated to comments

* lisp/ox.el (org-export-filters-alist): Remove reference to comment
  filters.
(org-export-filter-comment-functions,
org-export-filter-comment-block-functions): Remove variables.

* doc/org.texi (Advanced configuration): Remove reference to comment
  filters.

* etc/ORG-NEWS: Signal removal.

Comments are removed before parsing the buffer.  As a consequence,
filters are never called.

doc/org.texi

@@ -13588,57 +13588,55 @@ where @code{TYPE} is the type targeted by the filter.  Valid types are:
 @item center-block
 @tab clock
 @tab code
-@item comment
-@tab comment-block
-@tab diary-sexp
-@item drawer
+@item diary-sexp
+@tab drawer
 @tab dynamic-block
-@tab entity
-@item example-block
+@item entity
+@tab example-block
 @tab export-block
-@tab export-snippet
-@item final-output
+@item export-snippet
+@tab final-output
 @tab fixed-width
-@tab footnote-definition
-@item footnote-reference
+@item footnote-definition
+@tab footnote-reference
 @tab headline
-@tab horizontal-rule
-@item inline-babel-call
+@item horizontal-rule
+@tab inline-babel-call
 @tab inline-src-block
-@tab inlinetask
-@item italic
+@item inlinetask
+@tab italic
 @tab item
-@tab keyword
-@item latex-environment
+@item keyword
+@tab latex-environment
 @tab latex-fragment
-@tab line-break
-@item link
+@item line-break
+@tab link
 @tab node-property
-@tab options
-@item paragraph
+@item options
+@tab paragraph
 @tab parse-tree
-@tab plain-list
-@item plain-text
+@item plain-list
+@tab plain-text
 @tab planning
-@tab property-drawer
-@item quote-block
+@item property-drawer
+@tab quote-block
 @tab quote-section
-@tab radio-target
-@item section
+@item radio-target
+@tab section
 @tab special-block
-@tab src-block
-@item statistics-cookie
+@item src-block
+@tab statistics-cookie
 @tab strike-through
-@tab subscript
-@item superscript
+@item subscript
+@tab superscript
 @tab table
-@tab table-cell
-@item table-row
+@item table-cell
+@tab table-row
 @tab target
-@tab timestamp
-@item underline
+@item timestamp
+@tab underline
 @tab verbatim
-@tab verse-block
+@item verse-block
 @end multitable
 
 For example, the following snippet allows me to use non-breaking spaces in

etc/ORG-NEWS

@@ -125,6 +125,10 @@ MathJax automatically chooses the best display technology based on the
 end-users browser.  You may force initial usage of MathML via
 ~org-html-mathjax-template~ or by setting the ~path~ property of
 ~org-html-mathjax-options~.
+*** Removed comment-related filters
+~org-export-filter-comment-functions~ and
+~org-export-filter-comment-block-functions~ variables do not exist
+anymore.
 ** New features
 *** Hierarchies of tags
 The functionality of nesting tags in hierarchies is added to org-mode.

lisp/ox.el

@@ -189,8 +189,6 @@ way they are handled must be hard-coded into
     (:filter-center-block . org-export-filter-center-block-functions)
     (:filter-clock . org-export-filter-clock-functions)
     (:filter-code . org-export-filter-code-functions)
-    (:filter-comment . org-export-filter-comment-functions)
-    (:filter-comment-block . org-export-filter-comment-block-functions)
     (:filter-diary-sexp . org-export-filter-diary-sexp-functions)
     (:filter-drawer . org-export-filter-drawer-functions)
     (:filter-dynamic-block . org-export-filter-dynamic-block-functions)
@@ -2188,18 +2186,6 @@ Each filter is called with three arguments: the transcoded data,
 as a string, the back-end, as a symbol, and the communication
 channel, as a plist.  It must return a string or nil.")
 
-(defvar org-export-filter-comment-functions nil
-  "List of functions applied to a transcoded comment.
-Each filter is called with three arguments: the transcoded data,
-as a string, the back-end, as a symbol, and the communication
-channel, as a plist.  It must return a string or nil.")
-
-(defvar org-export-filter-comment-block-functions nil
-  "List of functions applied to a transcoded comment-block.
-Each filter is called with three arguments: the transcoded data,
-as a string, the back-end, as a symbol, and the communication
-channel, as a plist.  It must return a string or nil.")
-
 (defvar org-export-filter-diary-sexp-functions nil
   "List of functions applied to a transcoded diary-sexp.
 Each filter is called with three arguments: the transcoded data,