Revert of Add gpu implementation of OverdrawXfermode

src/gpu/glsl/GrGLSLXferProcessor.cpp

Index: src/gpu/glsl/GrGLSLXferProcessor.cpp
diff --git a/src/gpu/glsl/GrGLSLXferProcessor.cpp b/src/gpu/glsl/GrGLSLXferProcessor.cpp
index 9d2538f0818e791b2da4f18da213f011e202d680..81bd015718c8e8ac7aeea99b76998409d8c252b2 100644
--- a/src/gpu/glsl/GrGLSLXferProcessor.cpp
+++ b/src/gpu/glsl/GrGLSLXferProcessor.cpp
@@ -88,21 +88,3 @@
     this->onSetData(pdm, xp);
 }
 
-void GrGLSLXferProcessor::DefaultCoverageModulation(GrGLSLXPFragmentBuilder* fragBuilder,
-                                                    const char* srcCoverage,
-                                                    const char* dstColor,
-                                                    const char* outColor,
-                                                    const char* outColorSecondary,
-                                                    const GrXferProcessor& proc) {
-    if (proc.dstReadUsesMixedSamples()) {
-        if (srcCoverage) {
-            fragBuilder->codeAppendf("%s *= %s;", outColor, srcCoverage);
-            fragBuilder->codeAppendf("%s = %s;", outColorSecondary, srcCoverage);
-        } else {
-            fragBuilder->codeAppendf("%s = vec4(1.0);", outColorSecondary);
-        }
-    } else if (srcCoverage) {
-        fragBuilder->codeAppendf("%s = %s * %s + (vec4(1.0) - %s) * %s;",
-                                 outColor, srcCoverage, outColor, srcCoverage, dstColor);
-    }
-}