Use TLS for connector connections

The monitors and services didn't use TLS when they connected to the
backend servers. Since there has been no proof of instability, TLS should
be enabled.

server/core/mysql_utils.c

@@ -161,15 +161,7 @@ MYSQL *mxs_mysql_real_connect(MYSQL *con, SERVER *server, const char *user, cons
 
     if (listener)
     {
-#ifdef CONNECTOR_C_SSL_AND_OPENSSL_INTERFERENCE_SORTED_OUT
-        // TODO: No conclusive evidence yet, but tentatively it seems that when OpenSSL is
-        // TODO: used explicitly (backend SSL) and in conjunction with Connector-C, the
-        // TODO: latter SSL becomes unstable. So for the time being the monitors and
-        // TODO: services (fetch users) do not use SSL when connecting to the backend.
-
-        // mysql_ssl_set always returns true.
         mysql_ssl_set(con, listener->ssl_key, listener->ssl_cert, listener->ssl_ca_cert, NULL, NULL);
-#endif
     }
 
     return mysql_real_connect(con, server->name, user, passwd, NULL, server->port, NULL, 0);