Revert "Added --ffmpeg-<type>-opts to man page"

This reverts commit 7ff95ce42a0fadb112f7e7d9c8935f8bba42e2d6.
Removed --ffmpeg-<type>-opts

This feature isn't very useful with re-muxing in get_iplayer.  Keep it
simple and remove until someone comes up with use cases.