fix code generation for enum with generics

[nikitadol]

Sometimes you need to use enum with generics:

enum MyEnum<T> {
  ...
}

But in this case, the code generator generates the wrong code:

  static JsonTypeConverter2<MyEnum<Object?>, int, int> $convertername =
      const EnumIndexConverter<MyEnum<Object?>>(MyEnum<Object?>.values);

Since this part is not considered valid in Dart:

MyEnum<Object?>.values

Correct use:

MyEnum.values

This PR fixes this behavior

[simolus3]

Good catch and thanks for the fix, LGTM!