settings: Remove misleading and irrelevant comment.

This comment was _originally_ for the `default` memcached cache, back
when it was added all of the way back in 0a84d7a.  9e64750
made it a lie, and edc7189 made it even more confusing when it
removed the `default` cache configuration block, leaving the wrong
comment next to the wrong cache configuration block.

Banish the comment.

zproject/test_extra_settings.py

@@ -93,7 +93,6 @@
 # real app.
 USING_RABBITMQ = False
 
-# Disable use of memcached for caching
 CACHES["database"] = {
     "BACKEND": "django.core.cache.backends.dummy.DummyCache",
     "LOCATION": "zulip-database-test-cache",