WinGui: Remove the AV_MP4 option so we now only have the AV Format Muxer as "MP4 File".   The AV_MKV option will be removed at a later date.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5781 b64f7644-9d1e-0410-96f1-a4d463321fa5