avcodec/utils: Remove ff_codec_open2_recursive()

avcodec/utils: Remove ff_codec_open2_recursive()

This function existed to enable codecs with non-threadsafe init functions
to initialize other codecs despite the fact that normally no two codecs
with non-threadsafe init functions can be initialized at the same time
(there is a mutex guarding this). Yet there are no users of this
function any more as all users have been made thread-safe (switching
away from ff_codec_open2_recursive() was required for this as said
function requires the caller to hold the lock to the mutex guarding the
initializations and this is only true for codecs with the
FF_CODEC_CAP_INIT_THREADSAFE flag unset); so remove it.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>