Removing duplicated image generation code and using pre existing category.

I noticed that the code over here - https://github.com/wordpress-mobile/WordPress-iOS/blob/0489596182504e091d4ff803d01e5c7275233a68/WordPress/Classes/System/WordPressAppDelegate.m#L405-L415 was using code that we already have in a category that @aerych wrote sometime back. Just updating this portion of the code to use the pre existing category(https://github.com/wordpress-mobile/WordPress-iOS-Shared/blob/8679c488284d6ebb5ed5c826e487256fd4850eee/WordPress-iOS-Shared/UIImage%2BUtil.m#L5) to eliminate some duplication.