Remove unnecessary double test in Navigator

Summary:
_renderScene would unnecessary test if `this.state.transitionFromIndex != null` twice.

The left side and right side of the 'or' statement would always be the same.

For cleaner code I suggest to remove it.
Closes https://github.com/facebook/react-native/pull/10042

Differential Revision: D3923058

fbshipit-source-id: 7466c1f0f24eac3f9f296debd9a5e9f5320aea28

Libraries/CustomComponents/Navigator/Navigator.js

@@ -1248,7 +1248,7 @@ var Navigator = React.createClass({
         key={'scene_' + getRouteID(route)}
         ref={'scene_' + i}
         onStartShouldSetResponderCapture={() => {
-          return (this.state.transitionFromIndex != null) || (this.state.transitionFromIndex != null);
+          return (this.state.transitionFromIndex != null);
         }}
         pointerEvents={disabledScenePointerEvents}
         style={[styles.baseScene, this.props.sceneStyle, disabledSceneStyle]}>