lavc: replace internal use of AV_CODEC_CAP_AUTO_THREADS with an internal cap

lavc: replace internal use of AV_CODEC_CAP_AUTO_THREADS with an internal cap

AV_CODEC_CAP_AUTO_THREADS was originally added in b4d44a45f9a to mark
codecs that spawn threads internally and are able to select an optimal
threads count by themselves (all such codecs are wrappers around
external libraries). It is used by lavc generic code to check whether it
should handle thread_count=0 itself or pass the zero directly to the
codec implementation. Within this meaning, it is clearly supposed to be
an internal cap rather than a public one, since from the viewpoint of a
libavcodec user, lavc ALWAYS handles thread_count=0. Whether it happens
in the generic code or within the codec internals is not a meaningful
difference for the caller.

External aspects of this flag will be dealt with in the following
commit.