[Concurrency] Remove actorIndependent(unsafe) isolation.

Don't treat `actorIndependent(unsafe)` as its own kind of isolation.
It was only really used as a bring-up hack to break the isolation model,
but shouldn't be in the user model of the language and causes
complications to the implementation.

lib/IDE/CodeCompletion.cpp

@@ -2535,7 +2535,6 @@ public:
       break;
     case ActorIsolation::Unspecified:
     case ActorIsolation::Independent:
-    case ActorIsolation::IndependentUnsafe:
       return;
     }