Remove halfpix specialization

Remove halfpix specialization

This function only exists as a shortcut to subpixel variance with
predefined offsets. xoffset = 4 for horizontal, yoffset = 4 for vertical
and both for "hv"

Removing this allows the existing optimizations for the variance
functions to be called. Instead of having only sse2 optimizations, this
gives sse2, ssse3, msa and neon.

BUG=webm:1273

Change-Id: Ieb407b423b91b87d33c4263c6a1ad5e673b0efd6