Fix crash on double creation of listeners

When the listeners were created twice, MaxScale would crash when the users
were loaded. This is caused by the fact that the DCB for the listener is
NULL for failed listeners.

include/maxscale/service.h

@@ -162,7 +162,6 @@ typedef struct service
     bool strip_db_esc;                 /**< Remove the '\' characters from database names
                                         * when querying them from the server. MySQL Workbench seems
                                         * to escape at least the underscore character. */
-    SPINLOCK users_table_spin;         /**< The spinlock for users data refresh */
     SERVICE_REFRESH_RATE rate_limit;   /**< The refresh rate limit for users table */
     FILTER_DEF **filters;              /**< Ordered list of filters */
     int n_filters;                     /**< Number of filters */