remove unused suppressions in xplat

Summary:
Removes unused suppressions before deploying 0.91. See D13708161 for more context on why these couldn't be removed before.

I will follow up with 2 more diffs:
1. A diff that bumps the flow version in xplat and removes unused suppressions
2. A diff that adds new suppressions for 0.91

Reviewed By: samwgoldman

Differential Revision: D13720219

fbshipit-source-id: b07dd163962fed7ff27ce3e0a4a73a71c51965d9

Libraries/Components/ScrollView/ScrollView.js

@@ -761,7 +761,6 @@ class ScrollView extends React.Component<Props, State> {
   }
 
   _getKeyForIndex(index, childArray) {
-    // $FlowFixMe Invalid prop usage
     const child = childArray[index];
     return child && child.key;
   }