Clean up GF_GROUP data structure

Remove unused ref_frame_disp_idx and ref_frame_gop_idx.

BUG=aomedia:2728

Change-Id: I6e7e67c79b0ffab58abc541dd3d6765770d6309c

av1/encoder/firstpass.h

diff --git a/av1/encoder/firstpass.h b/av1/encoder/firstpass.h
index c4261db..6f85d48 100644
--- a/av1/encoder/firstpass.h
+++ b/av1/encoder/firstpass.h
@@ -175,8 +175,6 @@
   // frame.
   unsigned char cur_frame_idx[MAX_STATIC_GF_GROUP_LENGTH];
   unsigned char frame_disp_idx[MAX_STATIC_GF_GROUP_LENGTH];
-  int ref_frame_disp_idx[MAX_STATIC_GF_GROUP_LENGTH][REF_FRAMES];
-  int ref_frame_gop_idx[MAX_STATIC_GF_GROUP_LENGTH][REF_FRAMES];
 
   // TODO(jingning): Unify the data structure used here after the new control
   // mechanism is in place.