cb4x4: Move sub-4X4 TX sizes behind CONFIG_CHROMA_2X2.

cb4x4 itself should not require these sizes.

This simplifies compatibility with other experiments, since we can
first make them work with cb4x4 (which is now on by default), and
then worry about chroma_2x2 (which is not) in separate steps.

Encoder and decoder output should remain unchanged.

Change-Id: I4e9fcdae49f238b5099a3c74a398fe993c2545f8
20 files changed