Revert "ported from UXP: Issue #3092 - Implement BackgroundFinalizeTask for parallel garbage collection finalization (c0677633)"

This reverts commit fd2399f50d.

js/src/jsgc.cpp

@@ -4529,8 +4529,10 @@ PrepareWeakCacheTasks(JSRuntime* rt)
     WeakCacheTaskVector out;
     for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
         for (JS::WeakCache<void*>* cache : zone->weakCaches_) {
-            if (!out.emplaceBack(rt, *cache))
-                return out;
+            if (!out.append(SweepWeakCacheTask(rt, *cache))) {
+                SweepWeakCachesFromMainThread(rt);
+                return WeakCacheTaskVector();
+            }
         }
     }
     return out;