Remove the use case of bmi->as_mode

Remove the use case of bmi->as_mode in cb4x4 mode. Its function is
covered by 4x4 level mode_info.

Change-Id: I04abc1b7a0a97c12c3b6fddc1f16f7045512772e

av1/common/blockd.c

@@ -170,7 +170,11 @@ void av1_set_contexts(const MACROBLOCKD *xd, struct macroblockd_plane *pd,
   ENTROPY_CONTEXT *const l = pd->left_context + loff;
   const int txs_wide = tx_size_wide_unit[tx_size];
   const int txs_high = tx_size_high_unit[tx_size];
+#if CONFIG_CB4X4
+  const BLOCK_SIZE bsize = xd->mi[0]->mbmi.sb_type;
+#else
   const BLOCK_SIZE bsize = AOMMAX(xd->mi[0]->mbmi.sb_type, BLOCK_8X8);
+#endif
   const BLOCK_SIZE plane_bsize = get_plane_block_size(bsize, pd);
 
   // above

av1/common/blockd.h

@@ -353,7 +353,12 @@ typedef struct MODE_INFO {
 } MODE_INFO;
 
 static INLINE PREDICTION_MODE get_y_mode(const MODE_INFO *mi, int block) {
+#if CONFIG_CB4X4
+  (void)block;
+  return mi->mbmi.mode;
+#else
   return mi->mbmi.sb_type < BLOCK_8X8 ? mi->bmi[block].as_mode : mi->mbmi.mode;
+#endif
 }
 
 static INLINE int is_inter_block(const MB_MODE_INFO *mbmi) {