Yet another fix for the 5.5 branch

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -3561,7 +3561,6 @@ bool swift::checkSendableConformance(
   switch (getActorIsolation(nominal)) {
   case ActorIsolation::Unspecified:
   case ActorIsolation::ActorInstance:
-  case ActorIsolation::DistributedActorInstance:
   case ActorIsolation::Independent:
     break;