Remove SkBlitter::getShaderContext()

Unused.  Also ComposeShaderContext::getShaderContext{A,B}.

R=reed@google.com,mtklein@google.com

Change-Id: I36a8346f0c7e842455774339f388c04920db0040
Reviewed-on: https://skia-review.googlesource.com/7613
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed