Remove ASImageNode's tint property.

ASImageNodeTint is inflexible (your options are "no tint" and "use
`[UIColor grayColor]`") and needlessly complicates the ASImageNode
implementation.  Use ASImageNodeTintColorModificationBlock() instead.
Closes #383.

AsyncDisplayKit/ASImageNode.h

@@ -9,21 +9,6 @@
 #import <AsyncDisplayKit/ASControlNode.h>
 
 
-/**
- * Image tints.
- */
-typedef NS_ENUM(NSUInteger, ASImageNodeTint) {
-  /**
-   * No tint.
-   */
-  ASImageNodeTintNormal = 0,
-
-  /**
-   * Display the image in greyscale.
-   */
-  ASImageNodeTintGreyscale,
-};
-
 /**
  * Image modification block.  Use to transform an image before display.
  *
@@ -49,11 +34,6 @@ typedef UIImage *(^asimagenode_modification_block_t)(UIImage *image);
  */
 @property (atomic, retain) UIImage *image;
 
-/**
- * @abstract Simple way to tint the image.
- */
-@property (nonatomic, assign) ASImageNodeTint tint;
-
 /**
  @abstract The placeholder color.
  */