omero.gateway.BlitzGateway.__del__ hangs server

*Problem*: having any blocking actions in a __del__ method
can lead to hung gunicorn processes in OMERO.web. The addition
of the method was intended to detect dangling services, but
each call left further resources, detectable with `lsof`.

*Short-term fix*: By removing the method, OMERO.web should
no longer need to be periodically restarted.

*Long-term fix*: as a next step, the `_assert_unregistered`
method will need to again be invoked, perhaps by integration
tests, to detect the resources that were being left open.
Eventually, a rewrite of the `login_requred` decorator as
well as the `close` logic of BlitzGateway should be considered
so that resource cleanup can be guaranteed.

components/tools/OmeroPy/src/omero/gateway/__init__.py

@@ -1921,10 +1921,6 @@ def close(self):  # pragma: no cover
         self._proxies = NoProxies()
         logger.info("closed connecion (uuid=%s)" % str(self._sessionUuid))
 
-    def __del__(self):
-        logger.debug("##GARBAGE COLLECTOR KICK IN")
-        self._assert_unregistered()
-
     def _createProxies(self):
         """
         Creates proxies to the server services. Called on connection or