Fix #620, Remove legacy / non-unobtrusive client side validation
- remove `ViewContext.UnobtrusiveJavaScriptEnabled` property and all references
- avoid `ViewContext.GetFormContextForClientValidation()` calls since
  ternary expression is more explicit and we were inconsistent
- improve `ValidationMessage()` comments
- don't treat `ModelState.IsValid` as if it were still nullable
Cleanup unused `FormContext` members due to legacy validation removal
- provide new property back for customer use
- remove unused FieldValidationMetadataClass