Stopped trying to cache the currentScaleFactor

Was running into issues where the scale factor would get cleared when setting a new atributedString on a textNode.

I was clearing out the currentScaleFactor when setting an attributedString into a textNode. It appears that `_calculateSize` isn't always called when setting a new string into a ASTextNode. It can be the case that only `drawInContext:bounds:` is called. With _currentScaleFactor cleared out the renderer that calls`drawInContext...` was being called with a scaleFactor of 0.

It could be the case that the fix could be to remove the clearing of `currentScaleFactor` from `setAttributedString`, but this seems like a safer fix to me. It does, however, require an extra run through the font adjuster when enabled.

AsyncDisplayKit/TextKit/ASTextKitAttributes.h

@@ -86,10 +86,6 @@ struct ASTextKitAttributes {
    An array of scale factors in descending order to apply to the text to try to make it fit into a constrained size.
    */
   NSArray *pointSizeScaleFactors;
-  /**
-   The currently applied scale factor. Only valid if pointSizeScaleFactors are provided. Defaults to 0 (no scaling)
-   */
-  CGFloat currentScaleFactor;
   /**
    An optional block that returns a custom layout manager subclass. If nil, defaults to NSLayoutManager.
    */
@@ -123,7 +119,6 @@ struct ASTextKitAttributes {
       shadowOpacity,
       shadowRadius,
       pointSizeScaleFactors,
-      currentScaleFactor,
       layoutManagerCreationBlock,
       layoutManagerDelegate,
       textStorageCreationBlock,
@@ -138,7 +133,6 @@ struct ASTextKitAttributes {
     && shadowOpacity == other.shadowOpacity
     && shadowRadius == other.shadowRadius
     && [pointSizeScaleFactors isEqualToArray:other.pointSizeScaleFactors]
-    && currentScaleFactor == currentScaleFactor
     && layoutManagerCreationBlock == other.layoutManagerCreationBlock
     && textStorageCreationBlock == other.textStorageCreationBlock
     && CGSizeEqualToSize(shadowOffset, other.shadowOffset)