Kill shouldItemUpdate

Summary:
It was just adding unnecessary complexity. Users should just use standard React perf best practices, like `PureComponent` and `shouldComponentUpdate`.

This should be backwards compatible - existing `shouldItemUpdate` usage will just be ignored and should consider migrating to this pattern:

```
class MyItem extends React.PureComponent {
  _onPress = () => {
  	this.props.onPressItem(this.props.id);
  };
  render() {
    return (
      <SomeOtherWidget title={this.props.title} onPress={this._onPress} />
    )
  }
}
...
_renderItem = ({item}) => (
  <MyItem onPressItem={this._onPressItem} title={item.title} id={item.id} />
);
```

Which will automatically prevent re-renders unless the relavent data changes.

Reviewed By: yungsters

Differential Revision: D4730599

fbshipit-source-id: 0f61efe96eb4d95bb3b7c4ec889e3e0e34436e56

Examples/UIExplorer/js/MultiColumnExample.js

@@ -107,7 +107,6 @@ class MultiColumnExample extends React.PureComponent {
           onRefresh={() => alert('onRefresh: nothing to refresh :P')}
           refreshing={false}
           renderItem={this._renderItemComponent}
-          shouldItemUpdate={this._shouldItemUpdate}
           disableVirtualization={!this.state.virtualized}
           onViewableItemsChanged={this._onViewableItemsChanged}
           legacyImplementation={false}
@@ -127,11 +126,6 @@ class MultiColumnExample extends React.PureComponent {
       />
     );
   };
-  _shouldItemUpdate(prev, next) {
-    // Note that this does not check state.fixedHeight because we blow away the whole list by
-    // changing the key anyway.
-    return prev.item !== next.item;
-  }
   // This is called when items change viewability by scrolling into or out of the viewable area.
   _onViewableItemsChanged = (info: {
     changed: Array<{