Disabled prune_tx_type_est_rd speed feature on speed 6

Performance:
 TESTSET | AVG_PSNR | OVR_PSNR |   SSIM  |  SPD
  HDRES  |  +0.167% |  +0.185% | +0.177% | +4.6%
  MIDRES |  +0.111% |  +0.122% | +0.060% | +3.4%
  LOWRES |  +0.136% |  +0.150% | +0.156% | +4.4%

STATS_CHANGED

Change-Id: Ia5286e58184e2d6e48e42ec0dcfd062359a0a980

av1/encoder/speed_features.c

diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index 0495f4c..d273152 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -617,6 +617,7 @@
     sf->mv_sf.simple_motion_subpel_force_stop = FULL_PEL;
     sf->rd_sf.perform_coeff_opt = is_boosted_arf2_bwd_type ? 4 : 6;
     sf->tpl_sf.subpel_force_stop = FULL_PEL;
+    sf->tx_sf.tx_type_search.prune_tx_type_est_rd = 0;
   }
 }