Currently, http://ffmpeg.org/doxygen/trunk/group__preproc__misc.html is
broken, because of inclusion bug in Doxygen's preprocessing mode. The
now-unincluded header file is included anyway in avcodec.h, the only
place where it is used, so there should be no problem removing it.

One concern is API-compatibility, because old_codec_ids.h is a public
header. However, IMO this is not a problem because currently users
cannot include only this header and not `avcodec.h` anyway, because of
missing AV_CODEC_ID_NONE symbol.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>