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/gpu/effects/GrCustomXfermode.cpp b/src/gpu/effects/GrCustomXfermode.cpp
index e4d65e4..8d8f9f2 100644
--- a/src/gpu/effects/GrCustomXfermode.cpp
+++ b/src/gpu/effects/GrCustomXfermode.cpp
@@ -608,11 +608,10 @@
 }
 
 GrXferProcessor*
-GrCustomXPFactory::onCreateXferProcessor(const GrDrawTargetCaps& caps,
-                                         const GrProcOptInfo& colorPOI,
+GrCustomXPFactory::onCreateXferProcessor(const GrProcOptInfo& colorPOI,
                                          const GrProcOptInfo& coveragePOI,
                                          const GrDeviceCoordTexture* dstCopy) const {
-    return CustomXP::Create(fMode, dstCopy, this->willReadDstColor(caps, colorPOI, coveragePOI));
+    return CustomXP::Create(fMode, dstCopy, this->willReadDstColor(colorPOI, coveragePOI));
 }