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>
diff --git a/public.bzl b/public.bzl
index 4e10012..dee0fb8 100644
--- a/public.bzl
+++ b/public.bzl
@@ -602,6 +602,7 @@
     "SK_SUPPORT_LEGACY_ACCESSBITMAP",
     "SK_SUPPORT_LEGACY_CANVAS_IS_REFCNT",
     "SK_SUPPORT_LEGACY_CLIP_REGIONOPS",
+    "SK_SUPPORT_LEGACY_XFERMODE_IS_PUBLIC",
     # Temporarily Disable analytic AA for Google3
     "SK_NO_ANALYTIC_AA",
 ]