Allow rectangular coding block to search txfm sizes in speed 6

The encoding speed slow down in speed 6 is less than 5%. The
compression gains recouped are 0.84% in both PSNR and SSIM.

BUG=aomedia:2959

STATS_CHANGED

Change-Id: I7407ac74f4beea3e15dbd4505157248d63c1e6ce
diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index 0847bab..1ea410b 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -668,9 +668,8 @@
 
     sf->tx_sf.tx_type_search.winner_mode_tx_type_pruning = 2;
     sf->tx_sf.tx_type_search.prune_tx_type_est_rd = 0;
-    // Use largest txfm block size.
+    // Use largest txfm block size for square coding blocks.
     sf->tx_sf.intra_tx_size_search_init_depth_sqr = 2;
-    sf->tx_sf.intra_tx_size_search_init_depth_rect = 2;
     sf->tx_sf.tx_type_search.use_reduced_intra_txset = 2;
 
     sf->rd_sf.perform_coeff_opt = 6;