Revert of Use dst copies in porter duffer XP to correctly render certain blends. (patchset #4 id:60001 of https://codereview.chromium.org/914003003/)

Reason for revert:
Failing GLProgramTest passing in stupid coeffs

Original issue's description:
> Use dst copies in porter duffer XP to correctly render certain blends.
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/997c6358d94e188b1a7b89a4f86e24cbe0f5a164

TBR=bsalomon@google.com,joshualitt@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/923153003
diff --git a/src/effects/SkArithmeticMode_gpu.h b/src/effects/SkArithmeticMode_gpu.h
index 546902d..822e7bd 100644
--- a/src/effects/SkArithmeticMode_gpu.h
+++ b/src/effects/SkArithmeticMode_gpu.h
@@ -96,13 +96,11 @@
 private:
     GrArithmeticXPFactory(float k1, float k2, float k3, float k4, bool enforcePMColor); 
 
-    GrXferProcessor* onCreateXferProcessor(const GrDrawTargetCaps& caps,
-                                           const GrProcOptInfo& colorPOI,
+    GrXferProcessor* onCreateXferProcessor(const GrProcOptInfo& colorPOI,
                                            const GrProcOptInfo& coveragePOI,
                                            const GrDeviceCoordTexture* dstCopy) const SK_OVERRIDE; 
 
-    bool willReadDstColor(const GrDrawTargetCaps& caps,
-                          const GrProcOptInfo& colorPOI,
+    bool willReadDstColor(const GrProcOptInfo& colorPOI,
                           const GrProcOptInfo& coveragePOI) const SK_OVERRIDE {
         return true;
     }