Reduce time to clear existing world with lots of redirectors

- Skip validation for package already vetted for unloading
  - From ~30h (estimated) to 5.4s

From dev-enterprise[at]7226706

#rb JeanMichel.Dignard
#jira UE-76977

#ROBOMERGE-OWNER: ben.marsh
#ROBOMERGE-AUTHOR: danny.couture
#ROBOMERGE-SOURCE: CL 7260059 in //UE4/Release-4.23/... via CL 7260061
#ROBOMERGE-BOT: BUILD (Main -> Dev-Build) (v371-7306989)

[CL 7334462 by danny couture in Dev-Build branch]

Engine/Source/Editor/UnrealEd/Private/EditorEngine.cpp

@@ -2159,6 +2159,12 @@ void UEditorEngine::Cleanse( bool ClearSelection, bool Redraw, const FText& Tran
 		{
 			UPackage* RedirectorPackage = RedirIt->GetOutermost();
 
+			if (PackagesToUnload.Find(RedirectorPackage))
+			{
+				// Package was already marked to unload
+				continue;
+			}
+
 			if (RedirectorPackage == GetTransientPackage())
 			{
 				RedirIt->ClearFlags(FlagsToClear);