Don't create a GXPFactory when blend is SrcOver

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 ee3e73abb0f63b89b2eb96dfc7232d6fcfea3b3a..b24d23a6e6f3b8e81018a78f7ccffa818d2d8c80 100644
--- a/src/effects/SkArithmeticMode_gpu.h
+++ b/src/effects/SkArithmeticMode_gpu.h
@@ -81,10 +81,6 @@ public:
         return new GrArithmeticXPFactory(k1, k2, k3, k4, enforcePMColor);
     }
 
-    bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const override {
-        return true;
-    }
-
     void getInvariantBlendedColor(const GrProcOptInfo& colorPOI,
                                   GrXPFactory::InvariantBlendedColor*) const override;