Turn on prune_tx_type_using_stats at speed 3

Turned on prune_tx_type_using_stats at speed 3 as well.
Borg test result:
         avg_psnr:   ovr_psnr:   ssim:   avg encoder speedup:
hdres:    0.103       0.094      0.035     3.0%
midres:   0.123       0.128     -0.005     3.8%

Change-Id: Ie56bd04a44f5f76f0aaab3b02d4f50afdd02c621
diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index 4a96b62..34b436b 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -201,9 +201,6 @@
     // trained and tuned on speed 1 and 2. We might get better performance if we
     // readjust them for speed 3 and 4.
     sf->simple_motion_search_split = cm->allow_screen_content_tools ? 1 : 2;
-
-    // TODO(yunqing): make this work for speed 3.
-    sf->tx_type_search.prune_tx_type_using_stats = 0;
   }
 
   if (speed >= 4) {