Mark params __unused or #pragma unused

Summary:
Motivation: reduce build noise that might worry new users or hide real problems.

This deals with four of the warnings currently in the iOS build. The `__unused` additions are standard and self-explanatory, following the style used elsewhere in RN. I've used `#pragma unused` to deal with parameters named by macros.

Tested by building and running the `UIExplorer` example app in XCode 8.2
Closes https://github.com/facebook/react-native/pull/11797

Differential Revision: D4396611

fbshipit-source-id: 728e9ebb94d147f7a2cbc674a25fe67e66e2e8b2

React/Views/RCTScrollViewManager.m

@@ -88,6 +88,7 @@ RCT_EXPORT_VIEW_PROPERTY(onScrollAnimationEnd, RCTDirectEventBlock)
 // that css-layout is always treating the contents of a scroll container as
 // overflow: 'scroll'.
 RCT_CUSTOM_SHADOW_PROPERTY(overflow, YGOverflow, RCTShadowView) {
+#pragma unused (json)
   view.overflow = YGOverflowScroll;
 }