avcodec/ac3enc: Don't overwrite AVCodecContext.ch_layout

This is unnecessary (the channel layout guessing code became
moot when the channel layouts were enforced generically)
and also dangerous, as a custom channel layout mapping
would leak in case one was used.

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

libavcodec/ac3enc.c

@@ -2224,9 +2224,6 @@ static av_cold int set_channel_info(AVCodecContext *avctx)
     s->has_surround =  s->channel_mode & 0x04;
 
     s->channel_map  = ac3_enc_channel_map[s->channel_mode][s->lfe_on];
-    if (s->lfe_on)
-        mask |= AV_CH_LOW_FREQUENCY;
-    av_channel_layout_from_mask(&avctx->ch_layout, mask);
 
     return 0;
 }