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 
Agree this comment could be more clear and better-placed but it's _close_ to correct.  (I also dislike the double negative of `!excludePropertyErrors` but that's another matter.)

One possible improvement would be a comment such as
``` C#
// If excludePropertyErrors is true, describe any validation issue with the current model in a single item.
// Otherwise, list individual property errors.
```
above creation of the `<ul/>` element.  The comment here should be something like
``` C#
// Inform jQuery validation that it should replace the list of property errors in this element after validation failure.
```

@pranavkm any further suggestions to improve the comments in this method?