Clean up dead xfermode opts code.

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:

Committed: https://skia.googlesource.com/skia/+/54f313ccb8eba45954fe0a45092433cbf739b053

[mtklein_C, 2015-07-17 19:24:15]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-17 19:24:32]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1230023011/20001

[commit-bot: I haz the power, 2015-07-17 19:31:48]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-17 19:31:49]

Dry run: This issue passed the CQ dry run.

[mtklein_C, 2015-07-20 14:06:57]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-07-20 14:07:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1230023011/20001

[commit-bot: I haz the power, 2015-07-20 14:14:23]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/54f313ccb8eba45954fe0a45092433cbf739b053