Remove check for tx64 in non-420 images

This check is not needed as the tx_sizes are automatically clipped to
\leq TX_32X32 for chroma plane.

BUG=aomedia:2767

Change-Id: I342631f3ef9337c8b906552adad6b12f95f4ae7c
diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 43c2704..65f1279 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -597,10 +597,6 @@
   if (img->d_w != ctx->cfg.g_w || img->d_h != ctx->cfg.g_h)
     ERROR("Image size must match encoder init configuration size");
 
-  if (img->fmt != AOM_IMG_FMT_I420 && !ctx->extra_cfg.enable_tx64) {
-    ERROR("TX64 can only be disabled on I420 images.");
-  }
-
   return AOM_CODEC_OK;
 }