Clean up dead xfermode opts code.

Now that SK_SUPPORT_LEGACY_XFERMODES is unused, tons of code becomes dead.

Nothing is needed in opts/ anymore for x86.
We still do runtime NEON detection, which just duplicates Sk4pxXfermode.

TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1230023011
10 files changed