This commit replaces the vp8_ prefixed subtract function with the
common vpx_subtract_block function. It removes redundant SIMD
optimization codes and unit tests.
Change-Id: I42e086c32c93c6125e452dcaa6ed04337fe028d9
This is the final commit in the series converting to the new RTCD
system. It removes the encoder csystemdependent files and the remaining
global function pointers that didn't conform to the old RTCD system.
Change-Id: I9649706f1bb89f0cbf431ab0e3e7552d37be4d8e
