MXKViewControllerHandling: Remove "destroy" method

`destroy` method was introduced as workaround to get rid of retain cycles from the outside of the class. This method can be removed by using weak references where needed.

Start by locating MXKViewControllerHandling, its destroy method, and the outside references that use it. Inspect those references for the retain-cycle workaround; done means the method and its callers are removed or replaced with weak references where needed.

Written by the indexing model from the issue text.