Stops MongoDB Backend from using BROKER_USE_SSL.

This change effectively disables SSL as set using BROKER_USE_SSL.  SSL support
should be functional through existing mechanisms, but requires doc changes and
testing.  I've documented this potential feature and the necessary work with
celery/celery#1974.

celery/backends/mongodb.py

@@ -92,7 +92,6 @@ class MongoBackend(BaseBackend):
             self.options = dict(config, **config.pop('options', None) or {})
 
             # Set option defaults
-            self.options.setdefault('ssl', self.app.conf.BROKER_USE_SSL)
             self.options.setdefault('max_pool_size', self.max_pool_size)
             self.options.setdefault('auto_start_request', False)