Remove obsolete comment about t2667 failing

It was fixed by scala#390 and the test was added back in scala#408.

Author: smarter

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

@@ -35,7 +35,7 @@ class ExtensionMethods extends MiniPhaseTransform with DenotTransformer with Ful
     case ref: ClassDenotation if ref is ModuleClass =>
       ref.linkedClass match {
         case origClass: ClassSymbol if isDerivedValueClass(origClass) =>
-          val cinfo = ref.classInfo // ./tests/pos/t2667.scala dies here for module class AnyVal$
+          val cinfo = ref.classInfo
           val decls1 = cinfo.decls.cloneScope
           ctx.atPhase(thisTransformer.next) { implicit ctx =>
             for (decl <- origClass.classInfo.decls) {