remove discussion cache, which isn't used

cms/one_time_startup.py

@@ -1,11 +1,10 @@
 from dogapi import dog_http_api, dog_stats_api
 from django.conf import settings
 from xmodule.modulestore.django import modulestore
-from cache_toolbox.discussion_cache import discussion_cache_register_for_updates
 from django.dispatch import Signal
 from request_cache.middleware import RequestCache
 
-from django.core.cache import get_cache, InvalidCacheBackendError
+from django.core.cache import get_cache
 
 cache = get_cache('mongo_metadata_inheritance')
 for store_name in settings.MODULESTORE:
@@ -13,12 +12,8 @@ for store_name in settings.MODULESTORE:
     store.metadata_inheritance_cache_subsystem = cache
     store.request_cache = RequestCache.get_request_cache()
 
-    modulestore_update_signal = Signal(
-                            providing_args=['modulestore', 'course_id', 'location']
-                        )
+    modulestore_update_signal = Signal(providing_args=['modulestore', 'course_id', 'location'])
     store.modulestore_update_signal = modulestore_update_signal
-    discussion_cache_register_for_updates(store)
-
 if hasattr(settings, 'DATADOG_API'):
     dog_http_api.api_key = settings.DATADOG_API
     dog_stats_api.start(api_key=settings.DATADOG_API, statsd=True)