chore: remove ENUM_INDEX from facade

The ENUM_INDEX utility was added to return the index of an enum
consistently between Dart and TypeScript, so that the index
could be used to look up the name of the enum. Since dart is no
longer supported by Http, and since no other part of the framework
is using this function, it has been removed.

Closes #3843

modules/angular2/src/core/facade/lang.dart

@@ -13,8 +13,6 @@ class Math {
   static double random() => _random.nextDouble();
 }
 
-int ENUM_INDEX(value) => value.index;
-
 class CONST {
   const CONST();
 }