rate_limit: Remove inaccurate comment in rate_limit decorator.

The data is now stored in memory if things are happening inside tornado.
That aside, there is no reason for a comment on a rate_limit_user call
to talk about low level implementation details of that function.

zerver/decorator.py

@@ -796,7 +796,6 @@ def rate_limit(domain: str='api_by_user') -> Callable[[ViewFuncT], ViewFuncT]:
                 # TODO: implement per-IP non-authed rate limiting
                 return func(request, *args, **kwargs)
 
-            # Rate-limiting data is stored in redis
             rate_limit_user(request, user, domain)
 
             return func(request, *args, **kwargs)