xan: remove a trivially true if().

This function used to be shared with the xxan decoder, but that is not
true anymore.

libavcodec/xan.c

@@ -499,10 +499,9 @@ static int xan_decode_frame(AVCodecContext *avctx,
     const uint8_t *buf = avpkt->data;
     int ret, buf_size = avpkt->size;
     XanContext *s = avctx->priv_data;
-
-    if (avctx->codec->id == AV_CODEC_ID_XAN_WC3) {
     const uint8_t *buf_end = buf + buf_size;
     int tag = 0;
+
     while (buf_end - buf > 8 && tag != VGA__TAG) {
         unsigned *tmpptr;
         uint32_t new_pal;
@@ -554,7 +553,7 @@ static int xan_decode_frame(AVCodecContext *avctx,
         }
     }
     buf_size = buf_end - buf;
-    }
+
     if (s->palettes_count <= 0) {
         av_log(s->avctx, AV_LOG_ERROR, "No palette found\n");
         return AVERROR_INVALIDDATA;