rtc: set prune search intrabc speed feature

For speed < 7, realtime, screen mode:
mv_sf->enable prune_intrabc_candidate_block_hash_search.

Stats change for speed 6, rtc_screen, --enable-intrabc=1:
~0.3 bdrate loss, ~4% speedup.

Change-Id: I1b82c967f5f630c878d4e6b9a6691f0f6c5e59bb

av1/encoder/speed_features.c

diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index 03bf520..fb7b817 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -1766,6 +1766,7 @@
       sf->rt_sf.rt_use_intrabc = 1;
       sf->mv_sf.intrabc_search_level = 1;
       sf->mv_sf.hash_max_8x8_intrabc_blocks = 1;
+      sf->mv_sf.prune_intrabc_candidate_block_hash_search = 1;
     }
     if (speed >= 7) {
       sf->rt_sf.reduce_mv_pel_precision_highmotion = 0;