remove self-resizing of content view

DTAttributedTextContentView no longer self-resizes due to layout changes. If you are using it directly without DTAttributedTextView (which adds scrolling) then you have to first set the attributedString and then you have two options to know the optimal frame:

- You have subscribed to DTAttributedTextContentViewDidFinishLayoutNotification which communicates the optimal frame rect in the OptimalFrame value of the userInfo
- You call sizeToFit. This gets the size from sizeThatFits: which in turn gets it from intrinsicContentSize.

closes #102