make SkXfermode.h go away

make SkXfermode.h go away

This is step one:
- make SkXfermode useless to public clients
- everything they should need is in SkBlendMode.h

Step two:
- remove SkXfermode.h entirely (since skia core will already be using SkXfermodePriv.h)

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4534

Change-Id: If2cea9f71df92430ed6644edb98dd306c5572cbc
Reviewed-on: https://skia-review.googlesource.com/4534
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>