Remove unreachable default clauses from world_test.dart.

I'm currently working on a fix to the analyzer that will cause
unreachable default clauses to be properly flagged as a warning
(#54575). To avoid a bot
failure when the fix lands, these unreachable default clauses need to
be removed.

Bug: #54575
Change-Id: I35cc2be79d918a8f6ee7b388cf97044f0196b932
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/380961
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Nate Biggs <[email protected]>
Auto-Submit: Paul Berry <[email protected]>

pkg/compiler/test/model/world_test.dart

@@ -587,7 +587,6 @@ testCommonSubclasses() async {
       case SimpleSubclassResult.subtype2:
         return ClassQuery.SUBTYPE;
       case SetSubclassResult():
-      default:
         return null;
     }
   }
@@ -610,7 +609,6 @@ testCommonSubclasses() async {
       case SimpleSubclassResult.subtype2:
         return cls2;
       case SetSubclassResult():
-      default:
         return null;
     }
   }