remove xfermode flag (again) for g3

BUG=skia:

Change-Id: Ib6842553ff5e8b3003769ac352287dc0e0f7e691
NOTRY=True
Reviewed-on: https://skia-review.googlesource.com/5222
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/public.bzl b/public.bzl
index 970d756..be42ea0 100644
--- a/public.bzl
+++ b/public.bzl
@@ -600,7 +600,6 @@
     # Staging flags for API changes
     "SK_SUPPORT_LEGACY_ACCESSBITMAP",
     "SK_SUPPORT_LEGACY_CLIP_REGIONOPS",
-    "SK_SUPPORT_LEGACY_XFERMODE_IS_PUBLIC",
     "SK_SUPPORT_LEGACY_IMAGE_ENCODER_CLASS",
     # Temporarily Disable analytic AA for Google3
     "SK_NO_ANALYTIC_AA",