core: drop n_in_gc_queue field of Manager structure

We count the units in the GC queue with this, but actually never make use of
it, hence drop it.

src/core/manager.c

@@ -1015,8 +1015,6 @@ static unsigned manager_dispatch_gc_queue(Manager *m) {
                 }
         }
 
-        m->n_in_gc_queue = 0;
-
         return n;
 }
 

src/core/manager.h

@@ -229,7 +229,6 @@ struct Manager {
         int pin_cgroupfs_fd;
 
         int gc_marker;
-        unsigned n_in_gc_queue;
 
         /* Flags */
         ManagerExitCode exit_code:5;

src/core/unit.c

@@ -391,8 +391,6 @@ void unit_add_to_gc_queue(Unit *u) {
 
         LIST_PREPEND(gc_queue, u->manager->gc_queue, u);
         u->in_gc_queue = true;
-
-        u->manager->n_in_gc_queue++;
 }
 
 void unit_add_to_dbus_queue(Unit *u) {
@@ -570,10 +568,8 @@ void unit_free(Unit *u) {
         if (u->in_cleanup_queue)
                 LIST_REMOVE(cleanup_queue, u->manager->cleanup_queue, u);
 
-        if (u->in_gc_queue) {
+        if (u->in_gc_queue)
                 LIST_REMOVE(gc_queue, u->manager->gc_queue, u);
-                u->manager->n_in_gc_queue--;
-        }
 
         if (u->in_cgroup_queue)
                 LIST_REMOVE(cgroup_queue, u->manager->cgroup_queue, u);