Cannot drop a database with a maintenance daemon without the citus extension

If another database has a maintenance daemon, it can only dropped from a database with the Citus extension.

```
$ psql template1 -c "CREATE DATABASE foo"
$ psql foo -c "CREATE EXTENSION citus"
$ psql template1 -c "DROP DATABASE foo"
ERROR: database "foo" is being accessed by other users
DETAIL: There is 1 other session using the database.
```

The reason is that the `StopMaintenanceDaemon` function is only called from the utility hook if the Citus extension exists: https://github.com/citusdata/citus/blob/master/src/backend/distributed/executor/multi_utility.c#L464