Remove multiple coefficient buffers from PICK_MODE_CONTEXT

This commit is a manual cherry-pick from aom/master:
45592a39d3b00aee4d6bd70da669400017b7a5d8

Only part of the changes apply in nextgenv2

Change-Id: I1e22514c6fe5af556710254278f2f8a5805db999
4 files changed