Remove canTweakAlphaForCoverage from XP's since batch reads flag.

src/gpu/effects/GrCustomXfermodePriv.h

Index: src/gpu/effects/GrCustomXfermodePriv.h
diff --git a/src/gpu/effects/GrCustomXfermodePriv.h b/src/gpu/effects/GrCustomXfermodePriv.h
index 216fb62d43b164e74b1734f6d774e5fccffe34cb..edcdcc0a6e014f33ba8009c0860884c4b7849153 100644
--- a/src/gpu/effects/GrCustomXfermodePriv.h
+++ b/src/gpu/effects/GrCustomXfermodePriv.h
@@ -64,10 +64,6 @@ public:
         return true;
     }
 
-    bool canTweakAlphaForCoverage() const override {
-        return false;
-    }
-
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;