clean up SK_SUPPORT_LEGACY_A8_MASKBLITTER

cannot find anyone defining it

Change-Id: Ia93555b4d8fa1c07dd932dc39fe8854518f1455e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207720
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed