Resolved warnings in Quotes sample

Quotes/PagePreview.cs

@@ -5,7 +5,6 @@ using UIKit;
 using CoreFoundation;
 using Foundation;
 using CoreText;
-using CoreGraphics;
 
 namespace Quotes
 {

Quotes/PageView.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using CoreGraphics;
 using System.Linq;
 using UIKit;
 using Foundation;