avcodec/audiotoolboxenc: remove CAP_VARIABLE_FRAME_SIZE from alac

AV_CODEC_CAP_VARIABLE_FRAME_SIZE has been set for alac_at encoder,
which means avctx->frame_size should be zero. However, alac_at
encoder also set avctx->frame_size. This leading to assert failure
in ffmpeg_sched.c
	av_assert0(enc->sq_idx[0] >= 0);

Actually, the implementation of audiotoolboxenc.c doesn't support
frame_size been zero.

Fix #10720.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavcodec/audiotoolboxenc.c

@@ -676,7 +676,7 @@ static const uint64_t aac_at_channel_layouts[] = {
 
 FFAT_ENC(aac,          AV_CODEC_ID_AAC,          aac_profiles, , aac_at_channel_layouts, aac_at_ch_layouts)
 //FFAT_ENC(adpcm_ima_qt, AV_CODEC_ID_ADPCM_IMA_QT, NULL)
-FFAT_ENC(alac,         AV_CODEC_ID_ALAC,         NULL, | AV_CODEC_CAP_VARIABLE_FRAME_SIZE, NULL, NULL)
+FFAT_ENC(alac,         AV_CODEC_ID_ALAC,         NULL, , NULL, NULL)
 FFAT_ENC(ilbc,         AV_CODEC_ID_ILBC,         NULL, , NULL, NULL)
 FFAT_ENC(pcm_alaw,     AV_CODEC_ID_PCM_ALAW,     NULL, , NULL, NULL)
 FFAT_ENC(pcm_mulaw,    AV_CODEC_ID_PCM_MULAW,    NULL, , NULL, NULL)