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 
I'm not super sure this comment actually makes sense. The errors are already written out to the div if they're present