Don't create a GXPFactory when blend is SrcOver

src/gpu/effects/GrCustomXfermode.cpp

Index: src/gpu/effects/GrCustomXfermode.cpp
diff --git a/src/gpu/effects/GrCustomXfermode.cpp b/src/gpu/effects/GrCustomXfermode.cpp
index 6b3a20ec2d6e1d108989a9c0d566da00fc01c1c1..90ab030d2479807054f99caeda8710ccda2e70a5 100644
--- a/src/gpu/effects/GrCustomXfermode.cpp
+++ b/src/gpu/effects/GrCustomXfermode.cpp
@@ -321,10 +321,6 @@ class CustomXPFactory : public GrXPFactory {
 public:
     CustomXPFactory(SkXfermode::Mode mode);
 
-    bool supportsRGBCoverage(GrColor knownColor, uint32_t knownColorFlags) const override {
-        return true;
-    }
-
     void getInvariantBlendedColor(const GrProcOptInfo& colorPOI,
                                   GrXPFactory::InvariantBlendedColor*) const override;