Remove check for duplicates in module val names.

They are not lifted and they are allowed.

Author: DarkDimius

src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -67,12 +67,8 @@ class TreeChecker extends Phase with SymTransformer {
         printError(s"$sym has no superclass set")
 
       testDuplicate(sym, seenClasses, "class")
-
-    } else if (sym is ModuleVal) {
-      testDuplicate(sym, seenModuleVals, "module val")
     }
 
-
     symd
   }