cgroup: drop "ignore_self" argument from cg_is_empty()

In all cases where the function (or cg_is_empty_recursive()) ignoring
the calling process is actually wrong, as a process keeps a cgroup busy
regardless if its the current one or another. Hence, let's simplify
things and drop the "ignore_self" parameter.

src/machine/machinectl.c

@@ -375,7 +375,7 @@ static int show_unit_cgroup(sd_bus *bus, const char *unit, pid_t leader) {
         if (r < 0)
                 return bus_log_parse_error(r);
 
-        if (cg_is_empty_recursive(SYSTEMD_CGROUP_CONTROLLER, cgroup, false) != 0 && leader <= 0)
+        if (cg_is_empty_recursive(SYSTEMD_CGROUP_CONTROLLER, cgroup) != 0 && leader <= 0)
                 return 0;
 
         c = columns();