Allow non-square block size at smaller cb level

Disable non-square block size in partition at large coding block
level in speed 6. The all intra mode compression loss is 0.66% in
PSNR and 0.86% in SSIM. The encoding speed up is about 25%.

BUG=aomedia:2959

STATS_CHANGED

Change-Id: I57febe43a1de339d665b9d5a3507f1fd9659cbb3
diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index 7afba22..02deb3b 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -272,10 +272,7 @@
       sf->part_sf.default_min_partition_size = BLOCK_8X8;
     }
 
-    // TODO(yunqing): use BLOCK_32X32 for >= 4k.
     if (is_4k_or_larger) {
-      sf->part_sf.use_square_partition_only_threshold = BLOCK_64X64;
-    } else if (is_720p_or_larger) {
       sf->part_sf.use_square_partition_only_threshold = BLOCK_32X32;
     } else {
       sf->part_sf.use_square_partition_only_threshold = BLOCK_16X16;