avcodec/mpeg12: Don't initialize encoder-only parts of RLTable

avcodec/mpeg12: Don't initialize encoder-only parts of RLTable

ff_mpeg12_init_vlcs() currently initializes index_run, max_level and
max_run of ff_rl_mpeg1/2; yet the only user of these fields is the
MPEG-1/2 encoder which already initializes these tables on its own.
So remove the initializations in ff_mpeg12_init_vlcs(); this also
simplifies making ff_mpeg12_init_vlcs() thread-safe.

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