Don't use complex blend mode for shapes gm

The darken blend mode introduced barriers on most platforms, which
interfered with batching.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2031113003

Review-Url: https://codereview.chromium.org/2031113003

gm/shapes.cpp

@@ -59,7 +59,6 @@ protected:
     SkISize onISize() override { return SkISize::Make(500, 500); }
 
     void onOnceBeforeDraw() override {
-        fPaint.setXfermodeMode(SkXfermode::kDarken_Mode);
         fPaint.setAntiAlias(fAntialias);
     }