Remove canTweakAlphaForCoverage from XP's since batch reads flag.

src/gpu/effects/GrDisableColorXP.h

Index: src/gpu/effects/GrDisableColorXP.h
diff --git a/src/gpu/effects/GrDisableColorXP.h b/src/gpu/effects/GrDisableColorXP.h
index 9bbe24d757c10f0d0a313e7485afa95dfbaea50f..f5fe4fd6f1a186aeba0ec768c1d99ee1ba416fb1 100644
--- a/src/gpu/effects/GrDisableColorXP.h
+++ b/src/gpu/effects/GrDisableColorXP.h
@@ -23,8 +23,6 @@ public:
         return true;
     }
 
-    bool canTweakAlphaForCoverage() const override { return true; }
-
     void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
                             GrXPFactory::InvariantOutput* output) const override {
         output->fBlendedColorFlags = 0;