org.texi: Remove reference to removed variable

* doc/org.texi (Plain lists): Remove reference to
  `org-list-empty-line-terminates-plain-lists'.

Reported-by: Alain.Cochard@unistra.fr
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-09/msg00603.html>

doc/org.texi

@@ -1658,11 +1658,9 @@ line.  In particular, if an ordered list reaches number @samp{10.}, then the
 list.  An item ends before the next line that is less or equally indented
 than its bullet/number.
 
-@vindex org-list-empty-line-terminates-plain-lists
 A list ends whenever every item has ended, which means before any line less
 or equally indented than items at top level.  It also ends before two blank
-lines@footnote{See also @code{org-list-empty-line-terminates-plain-lists}.}.
-In that case, all items are closed.  Here is an example:
+lines.  In that case, all items are closed.  Here is an example:
 
 @example
 @group