quantize: remove vp9_regular_quantize_b_4x4

This was just a helper function which called vpx_quantize_b or
vpx_highbd_quantize_b. It also checked for skip_block, which was
necessary when webm:1439 was filed but does not appear to be
necessary now.

Removes a quantize variant and makes subsequent cleanups easier.

Change-Id: Ibe545eccd19370f07ff26c8e151f290c642efd2a
3 files changed