Always use the 16-bit coding path

The encoder and decoder will now use the 16-bit (high-bitdepth)
coding path, regardless of the encoding profile.

This is a step towards the removal of the 8-bit (low-bitdepth)
coding path (which remains compile-checked at the moment).

BUG=aomedia:611

Change-Id: Ie764c39a327d48b18d77246a028493b6270b9f88

configure

@@ -507,9 +507,6 @@ 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