Fix use of non-existant APIs

I must've been thinking I was going to add a stop function at some point
in the future. Rather than do that I just used the cast I added
previously.

src/couch_event_listener_mfa.erl

@@ -52,7 +52,7 @@ enter_loop(Mod, Func, State, Options) ->
 
 
 stop(Pid) ->
-    couch_event_listener:stop(Pid).
+    couch_event_listener:cast(Pid, shutown).
 
 
 init({Parent, Mod, Func, State}) ->