This option is obsolete and confusing now that the LBD coding path
can be disabled at build time (--disable-lowbitdepth).
Removing it makes the encoder/decoder behaviours symmetric, and
prevents the "--test-16bit-internal --test-decode=fatal" mismatch.

BUG=aomedia:528

Change-Id: Ia2d9857629b789b11d37fc75433b2cecc27d6642