Disable selective_ref_frame = 4 temporarily

Speed feature selective_ref_frame has been temporarily disabled
for selective_ref_frame > 3.

BUG=aomedia:2318

Change-Id: I743628b69e367c30af9cee8b785aa238bef6b8c2
diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index a48b996..d4b0228 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -308,7 +308,6 @@
   }
 
   if (speed >= 3) {
-    sf->selective_ref_frame = 4;
     sf->tx_size_search_method = boosted ? USE_FULL_RD : USE_LARGESTALL;
     sf->less_rectangular_check_level = 2;
     sf->adaptive_pred_interp_filter = 1;