hide accessBitmap from g3

BUG=skia:

Change-Id: I00e8121751e640ee3b449d27b6ccfcc1491964a7
Reviewed-on: https://skia-review.googlesource.com/5224
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/public.bzl b/public.bzl
index be42ea0..8977e73 100644
--- a/public.bzl
+++ b/public.bzl
@@ -598,7 +598,6 @@
     # Turn on a few Google3-specific build fixes.
     "GOOGLE3",
     # Staging flags for API changes
-    "SK_SUPPORT_LEGACY_ACCESSBITMAP",
     "SK_SUPPORT_LEGACY_CLIP_REGIONOPS",
     "SK_SUPPORT_LEGACY_IMAGE_ENCODER_CLASS",
     # Temporarily Disable analytic AA for Google3