libvpx

Only support improved quant

Deprecate fast quant and strict_quant code.
Small effect on quality as fast was used in first pass but the
effect is basically neutral across the derf set.

The rationale here is to reduce the number of code paths for
now to make experimentation easier. Optimized and fast code
options can be re-introduced later along with other  encode
speed options.

Change-Id: Ia30c5daf3dbc52e72c83b277a1d281e3c934cdad