doc: remove wrong libvlc_media sub groups
doc: remove wrong libvlc_media sub groups

They never worked because of a missing '\' before defgroup.

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc