lavc: Drop exporting 2-pass encoding stats

lavc: Drop exporting 2-pass encoding stats

These variables are coming from mpegvideoenc where are supposedly used
as bit counters on various frame properties. However their use is
unclear as they lack documentation, are available only from a very small
subset of encoders, and they are hardly used in the wild. Also frame_bits
in aacenc is employed in a similar way.

Remove this functionality from AVCodecContex, these variable are mostly
frame properties, and too few encoders support setting them with anything
useful.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>