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/scope.c

@@ -171,7 +171,7 @@ static int scope_load(Unit *u) {
         return scope_verify(s);
 }
 
-static int scope_coldplug(Unit *u, Hashmap *deferred_work) {
+static int scope_coldplug(Unit *u) {
         Scope *s = SCOPE(u);
         int r;