cgroup-util: drop 'controller' argument from cg_get_keyed_attribute() and cg_is_empty()

The argument is completely unused. Let's drop it.

src/machine/machine.c

@@ -596,7 +596,7 @@ static int machine_dispatch_cgroup_empty(sd_event_source *s, const struct inotif
 
         assert(m->cgroup);
 
-        r = cg_is_empty(SYSTEMD_CGROUP_CONTROLLER, m->cgroup);
+        r = cg_is_empty(m->cgroup);
         if (r < 0)
                 return log_error_errno(r, "Failed to determine if cgroup '%s' is empty: %m", m->cgroup);
 
@@ -782,7 +782,7 @@ bool machine_may_gc(Machine *m, bool drop_not_started) {
         }
 
         if (m->cgroup) {
-                r = cg_is_empty(SYSTEMD_CGROUP_CONTROLLER, m->cgroup);
+                r = cg_is_empty(m->cgroup);
                 if (IN_SET(r, 0, -ENOENT))
                         return true;
                 if (r < 0)

src/machine/machinectl.c

@@ -452,7 +452,7 @@ static int show_unit_cgroup(
 
                 /* Fallback for older systemd versions where the GetUnitProcesses() call is not yet available */
 
-                if (cg_is_empty(SYSTEMD_CGROUP_CONTROLLER, cgroup) != 0 && leader <= 0)
+                if (cg_is_empty(cgroup) != 0 && leader <= 0)
                         return 0;
 
                 show_cgroup_and_extra(cgroup, "\t\t  ", c, &leader, leader > 0, get_output_flags());