Revert "core: do not spawn jobs or touch other units during coldplugging"

This reverts commit 6e392c9c45.

We really shouldn't invent external state keeping hashmaps, if we can
keep this state in the units themselves.

src/core/slice.c

@@ -150,7 +150,7 @@ static int slice_load(Unit *u) {
         return slice_verify(s);
 }
 
-static int slice_coldplug(Unit *u, Hashmap *deferred_work) {
+static int slice_coldplug(Unit *u) {
         Slice *t = SLICE(u);
 
         assert(t);