system_keyspace: call shutdown from stop

and use that to replace the explicit shutdown when stopped
in cql_test_env.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

test/lib/cql_test_env.cc

@@ -699,7 +699,6 @@ private:
 
             _sys_ks.start(std::ref(_qp), std::ref(_db)).get();
             auto stop_sys_kd = defer_verbose_shutdown("system keyspace", [this] {
-                _sys_ks.invoke_on_all(&db::system_keyspace::shutdown).get();
                 _sys_ks.stop().get();
             });