Remove left-over :export-block keywords

* contrib/lisp/ox-freemind.el (freemind):
* contrib/lisp/ox-gfm.el (gfm):
* contrib/lisp/ox-groff.el (groff): Remove :export-block keyword.

* doc/org.texi (Adding export back-ends): Remove documentation about the
  keyword.

Reported-by: Kaushal Modi <kaushal.modi@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/104057>

contrib/lisp/ox-freemind.el

@@ -43,7 +43,6 @@
 ;;; Define Back-End
 
 (org-export-define-derived-backend 'freemind 'html
-  :export-block "FREEMIND"
   :menu-entry
   '(?f "Export to Freemind Mindmap"
        ((?f "As Freemind Mindmap file" org-freemind-export-to-freemind)

contrib/lisp/ox-gfm.el

@@ -51,7 +51,6 @@
 ;;; Define Back-End
 
 (org-export-define-derived-backend 'gfm 'md
-  :export-block '("GFM" "GITHUB FLAVORED MARKDOWN")
   :filters-alist '((:filter-parse-tree . org-md-separate-elements))
   :menu-entry
   '(?g "Export to Github Flavored Markdown"

contrib/lisp/ox-groff.el

@@ -92,7 +92,6 @@
     (underline . org-groff-underline)
     (verbatim . org-groff-verbatim)
     (verse-block . org-groff-verse-block))
-  :export-block "GROFF"
   :menu-entry
   '(?g "Export to GROFF"
        ((?g "As GROFF file" org-groff-export-to-groff)

doc/org.texi

@@ -17967,10 +17967,9 @@ back-end from an existing one.
 When creating a new back-end from scratch, the basic idea is to set the name
 of the back-end (as a symbol) and an alist of elements and export functions.
 On top of this, you will need to set additional keywords like
-@code{:menu-entry} (to display the back-end in the export dispatcher),
-@code{:export-block} (to specify what blocks should not be exported by this
-back-end), and @code{:options-alist} (to let the user set export options that
-are specific to this back-end.)
+@code{:menu-entry} (to display the back-end in the export dispatcher), and
+@code{:options-alist} (to let the user set export options that are specific
+to this back-end.)
 
 Deriving a new back-end is similar, except that you need to set
 @code{:translate-alist} to an alist of export functions that should be used