fix several lint warnings

Summary: fix some ling warnings from several files
Closes https://github.com/facebook/react-native/pull/4450

Reviewed By: svcscm

Differential Revision: D2707606

Pulled By: mkonicek

fb-gh-sync-id: 410ccacf061ac7b0f6e44d1a5e4621a9d1d606fc

Libraries/Network/XMLHttpRequest.ios.js

@@ -13,7 +13,6 @@
 
 var FormData = require('FormData');
 var RCTNetworking = require('RCTNetworking');
-var RCTDeviceEventEmitter = require('RCTDeviceEventEmitter');
 
 var XMLHttpRequestBase = require('XMLHttpRequestBase');
 

Libraries/Picker/PickerIOS.ios.js

@@ -15,13 +15,11 @@
 var NativeMethodsMixin = require('NativeMethodsMixin');
 var React = require('React');
 var ReactChildren = require('ReactChildren');
-var ReactNativeViewAttributes = require('ReactNativeViewAttributes');
 var RCTPickerIOSConsts = require('NativeModules').UIManager.RCTPicker.Constants;
 var StyleSheet = require('StyleSheet');
 var View = require('View');
 
 var requireNativeComponent = require('requireNativeComponent');
-var merge = require('merge');
 
 var PICKER = 'picker';
 

Libraries/RCTTest/SnapshotViewIOS.ios.js

@@ -11,7 +11,6 @@
  */
 'use strict';
 
-var Platform = require('Platform');
 var React = require('React');
 var StyleSheet = require('StyleSheet');
 var { TestModule, UIManager } = require('NativeModules');