Revert "Always use the 16-bit coding path"

This reverts commit 3d264da25815203ecd71a4f6e6855fe3f3edceb5.

This causes codecbase 30% compression performance regression.

BUG=aomedia:632

Change-Id: I84ce601b54cd48e06d2609f1de3e325e9fc08785

configure

diff --git a/configure b/configure
index 89245ef..04d5da0 100755
--- a/configure
+++ b/configure
@@ -516,6 +516,9 @@
     # 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