Stop dictprop deleting key if value is None

Currently, the DictProperty deletes a key if `__setitem__` is called with a value of 'None'. It isn't clear why it should do this, since normal python dictionaries would allow the assignation. The existing code even includes the comment '# is this really needed? '.

This pull request resolves the issue at https://github.com/kivy/kivy/issues/1461 by deleting the extra behaviour and letting the user set DictProperty values to None.