The fLaC and dfLa box IDs have been registered with the MP4 RA
(they are now listed at https://mp4ra.org/#/codecs) and support
for muxing FLAC in MP4 has been experimental in ffmpeg for
6 years now, since Nov 21, 2016

This patch removes the experimental status and removes the MP4
object type, as none has been registered for FLAC as it was not
deemed necessary.
Signed-off-by: Anton Khirnov <anton@khirnov.net>