Deregisters an existing key.

            `key`
                String key to deregister.

            Returns boolean.

def deregister(self, key):
        """ Deregisters an existing key.

            `key`
                String key to deregister.

            Returns boolean.
            """

        if not key in self._actions:
            return False

        del self._actions[key]

        if key in self._cache:
            del self._cache[key]

        return True