remove CANVAS_IS_REFCNT flag from public.bzl

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4914

Change-Id: I3ea6c2b96dd79da9ebba659726cbab52bda21651
Reviewed-on: https://skia-review.googlesource.com/4914
Reviewed-by: Mike Reed <reed@google.com>

public.bzl

diff --git a/public.bzl b/public.bzl
index dee0fb8..bf4137c 100644
--- a/public.bzl
+++ b/public.bzl
@@ -600,7 +600,6 @@
     "GOOGLE3",
     # Staging flags for API changes
     "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