Revert "Always use the 16-bit coding path"

This reverts commit 3d264da2.

This causes codecbase 30% compression performance regression.

BUG=aomedia:632

Change-Id: I84ce601b54cd48e06d2609f1de3e325e9fc08785

configure

@@ -516,6 +516,9 @@ post_process_cmdline() {
     # Backwards/jenkins compatibility with --enable-aom-highbitdepth
     enabled aom_highbitdepth && enable_feature highbitdepth
 
+    # Enable low-bitdepth pixel pipeline by default
+    soft_enable lowbitdepth
+
     soft_enable palette
     soft_enable alt_intra
     soft_enable palette_throughput