Remove unreachable default clauses from module_builder.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, this unreachable default clause needs to
be removed.

Bug: #54575
Change-Id: Iba0d32cf28f716f790c68a88b209420871845db8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/380963
Commit-Queue: Paul Berry <[email protected]>
Auto-Submit: Paul Berry <[email protected]>
Reviewed-by: Mark Zhou <[email protected]>

Author: stereotype441

pkg/dev_compiler/lib/src/compiler/module_builder.dart

@@ -88,7 +88,6 @@ Program transformModuleFormat(ModuleFormat format, Program module) {
     case ModuleFormat.amd:
       return AmdModuleBuilder().build(module);
     case ModuleFormat.es6:
-    default:
       return module;
   }
 }