doc/developer: remove merge request method of contributing

This seems to confuse Github users into thinking that we may accept pull
requests. We do not accept pull requests.

Sending patches to the ffmpeg-devel mailing list is our preferred method
for users to contribute code.

Signed-off-by: Lou Logan <lou@lrcd.com>

doc/developer.texi

@@ -30,13 +30,11 @@ consult @url{https://ffmpeg.org/legal.html}.
 
 @chapter Contributing
 
-There are 3 ways by which code gets into FFmpeg.
+There are 2 ways by which code gets into FFmpeg:
 @itemize @bullet
-@item Submitting patches to the main developer mailing list.
+@item Submitting patches to the ffmpeg-devel mailing list.
       See @ref{Submitting patches} for details.
 @item Directly committing changes to the main tree.
-@item Committing changes to a git clone, for example on github.com or
-      gitorious.org. And asking us to merge these changes.
 @end itemize
 
 Whichever way, changes should be reviewed by the maintainer of the code