avcodec/mpeg12dec: Don't call ff_mpeg12_common_init()

It only sets [yc]_dc_scale_table and these tables are only
read in ff_set_qscale(); but the MPEG-1/2 decoders don't call
ff_set_qscale() at all.
(Furthermore, given that intra_dc_precision is always zero
for a decoder at this point, ff_mpeg12_common_init()
actually set these pointers to what ff_mpv_common_defaults()
already set them.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/mpeg12dec.c

@@ -1062,7 +1062,6 @@ static av_cold int mpeg_decode_init(AVCodecContext *avctx)
     /* we need some permutation to store matrices,
      * until the decoder sets the real permutation. */
     ff_mpv_idct_init(s2);
-    ff_mpeg12_common_init(&s->mpeg_enc_ctx);
     ff_mpeg12_init_vlcs();
 
     s2->chroma_format              = 1;
@@ -3075,7 +3074,6 @@ static av_cold int ipu_decode_init(AVCodecContext *avctx)
 
     ff_mpv_decode_init(m, avctx);
     ff_mpv_idct_init(m);
-    ff_mpeg12_common_init(m);
     ff_mpeg12_init_vlcs();
 
     for (int i = 0; i < 64; i++) {