Remove canTweakAlphaForCoverage from XP's since batch reads flag.

src/effects/SkArithmeticMode_gpu.h

Index: src/effects/SkArithmeticMode_gpu.h
diff --git a/src/effects/SkArithmeticMode_gpu.h b/src/effects/SkArithmeticMode_gpu.h
index f9a3d35fcaccbf520d1deb65701d5e6b14255753..9f6953e2381a037f03de2535d844f8c467b599c6 100644
--- a/src/effects/SkArithmeticMode_gpu.h
+++ b/src/effects/SkArithmeticMode_gpu.h
@@ -81,10 +81,6 @@ public:
         return true;
     }
 
-    bool canTweakAlphaForCoverage() const override {
-        return false;
-    }
-
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput*) const override;