Fix #620, Remove legacy / non-unobtrusive client side validation

- remove `ViewContext.UnobtrusiveJavaScriptEnabled` property and all references
- also avoid `ViewContext.GetFormContextForClientValidation()` calls since
  ternary expression is more explicit and we were inconsistent

/cc @sornaks @yishaigalatzer @DamianEdwards 
`ModelState.IsValid` is a boolean property now. Should be fine saying 

`if (ViewData.ModelState.IsValid && (formContext == null || excludePropertyErrors))`