Clean up intra mode context fetch functions

Following the cb4x4 design, the logic related to sub8x8 handling
can be simplifed.

Change-Id: I1ed0c0327a4498614408320a583ad37607bde4af
diff --git a/av1/common/blockd.h b/av1/common/blockd.h
index 96bc17b..cda8f6f 100644
--- a/av1/common/blockd.h
+++ b/av1/common/blockd.h
@@ -432,11 +432,9 @@
 }
 #endif  // CONFIG_EXT_COMP_REFS
 
-PREDICTION_MODE av1_left_block_mode(const MODE_INFO *cur_mi,
-                                    const MODE_INFO *left_mi, int b);
+PREDICTION_MODE av1_left_block_mode(const MODE_INFO *left_mi);
 
-PREDICTION_MODE av1_above_block_mode(const MODE_INFO *cur_mi,
-                                     const MODE_INFO *above_mi, int b);
+PREDICTION_MODE av1_above_block_mode(const MODE_INFO *above_mi);
 
 static INLINE int is_global_mv_block(const MODE_INFO *mi,
                                      TransformationType type) {