Allow lxc_clear_config_item to clear idmaps.

Ran into this when trying to automate stgraber's "GUI in containers"
post.

Signed-off-by: Casey Marshall <casey.marshall@canonical.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>

src/lxc/confile.c

@@ -2241,6 +2241,9 @@ int lxc_clear_config_item(struct lxc_conf *c, const char *key)
 		lxc_seccomp_free(c);
 		return 0;
 	}
+	else if (strncmp(key, "lxc.id_map", 10) == 0) {
+		return lxc_clear_idmaps(c);
+	}
 
 	return -1;
 }