views: refactor away PlatformStyle::BackgroundColorForMdButton

ui/views/style/platform_style.cc

Index: ui/views/style/platform_style.cc
diff --git a/ui/views/style/platform_style.cc b/ui/views/style/platform_style.cc
index 76932f1fbeb7491654d145ae2bf98f9ab90b6dc7..e9a548384c07ebf68168ee1c2fd586f0491aab01 100644
--- a/ui/views/style/platform_style.cc
+++ b/ui/views/style/platform_style.cc
@@ -89,12 +89,6 @@ SkColor PlatformStyle::TextColorForButton(
   return color_by_state[button.state()];
 }
 
-SkColor PlatformStyle::BackgroundColorForMdButton(
-    SkColor color,
-    Button::ButtonState state) {
-  return color;
-}
-
 #endif  // OS_MACOSX
 
 #if !defined(DESKTOP_LINUX) && !defined(OS_MACOSX)