Stop expose Image.resizeMode

Summary:
Exposing this enum is essentially useless and at worst is a runtime cost that isn't necessary by just using the string.

The value of this enum, as far as I understand it, is to enforce that only valid options are used. We can enforce this at build time with Flow.

I was able to migrate our codebase with a few Find and Replace for things like

```
resizeMode={Image.resizeMode.contain}
```

Reviewed By: yungsters

Differential Revision: D7983982

fbshipit-source-id: ddd7024023f8d2f01aad1fff6c8103983a1bec1a

Libraries/Image/Image.android.js

@@ -10,7 +10,6 @@
 
 'use strict';
 
-const ImageResizeMode = require('ImageResizeMode');
 const ImageStylePropTypes = require('ImageStylePropTypes');
 const NativeMethodsMixin = require('NativeMethodsMixin');
 const NativeModules = require('NativeModules');
@@ -132,8 +131,6 @@ const Image = createReactClass({
   },
 
   statics: {
-    resizeMode: ImageResizeMode,
-
     getSize(
       url: string,
       success: (width: number, height: number) => void,