Removed deprecated zigbee.mqtt backend.

Closes: #322

platypush/backend/zigbee/mqtt/__init__.py

@@ -1,35 +0,0 @@
-import warnings
-
-from platypush.backend import Backend
-
-
-class ZigbeeMqttBackend(Backend):
-    """
-    Listen for events on a zigbee2mqtt service.
-
-    **WARNING**: This backend is **DEPRECATED** and it will be removed in a
-    future version.
-
-    It has been merged with
-    :class:`platypush.plugins.zigbee.mqtt.ZigbeeMqttPlugin`.
-
-    Now you can simply configure the `zigbee.mqtt` plugin in order to enable
-    the Zigbee integration - no need to enable both the plugin and the backend.
-    """
-
-    def run(self):
-        super().run()
-        warnings.warn(
-            '''
-            The zigbee.mqtt backend has been merged into the zigbee.mqtt
-            plugin. It is now deprecated and it will be removed in a future
-            version.
-            Please remove any references to it from your configuration.
-            ''',
-            DeprecationWarning,
-        )
-
-        self.wait_stop()
-
-
-# vim:sw=4:ts=4:et: