server: revert commit 7e6556b3ca

With the removal of old tpiu code, commit 7e6556b3ca ("server:
permit the add_service function to return the created service")
http://openocd.zylin.com/5717/ can be reverted.

Fix also the new calls to add_service().

Change-Id: Ib7f2dfc6a9e829239e20313e0f121911085fdc00
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6031
Tested-by: jenkins
Reviewed-by: Adrian M Negreanu <adrian.negreanu@nxp.com>

src/server/gdb_server.c

@@ -3509,7 +3509,7 @@ static int gdb_target_start(struct target *target, const char *port)
 
 	ret = add_service("gdb",
 			port, target->gdb_max_connections, &gdb_new_connection, &gdb_input,
-			&gdb_connection_closed, gdb_service, NULL);
+			&gdb_connection_closed, gdb_service);
 	/* initialize all targets gdb service with the same pointer */
 	{
 		struct target_list *head;