Remove the callback facility from context node.

Remove the callback facility from context node.

- It's not really necessary.
- It's probably a mis-use of QSGNode::preprocess()
  - Because it triggers a race between object deletion in the main thread
  and preprocess() in the render thread.

Change-Id: I51433f508db306aab53398f8c3d1d8c9d9e986fd
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>
Reviewed-by: Yunqiao Yin <charles.yin@nokia.com>