remove unused GM flags

gm/imagealphathreshold.cpp

Index: gm/imagealphathreshold.cpp
diff --git a/gm/imagealphathreshold.cpp b/gm/imagealphathreshold.cpp
index b2259949c6b412760e10844d8cd9e50cc21d73f7..a6b00e4c940b9c8297bda3ba4e18dfd0b4ed5480 100644
--- a/gm/imagealphathreshold.cpp
+++ b/gm/imagealphathreshold.cpp
@@ -21,13 +21,6 @@ public:
     }
 
 protected:
-    uint32_t onGetFlags() const SK_OVERRIDE {
-        return this->INHERITED::onGetFlags() |
-               GM::kSkipTiled_Flag |
-               GM::kSkipPicture_Flag |
-               GM::kSkipPipe_Flag |
-               GM::kSkipPipeCrossProcess_Flag;
-    }
 
     SkString onShortName() SK_OVERRIDE {
         return SkString("imagealphathreshold");