Remove legacy / non-unobtrusive client side validation

[pranavkm]

Mvc5 supported a non-unobtrusive way for performing client side validation. This was triggered when you set htmlhelper.unobtrusivejavascriptenabled to false. We need to figure out if we are still interested in supporting this ability going forward

[dougbu]

/cc @danroth27 @DamianEdwards this is a top-level question we need to answer in this milestone.

Sub-items such as #163, #435, and #449 would not be necessary if we decide only Unobtrusive validation (and the hooks to override the specific attributes generated) is necessary. Should also look at whether we need all FormContext properties should that be our choice. On the other hand, may need extensibility points allowing other non-Unobtrusive JavaScript to be included in or around a <form/> -- not just overriding the Unobtrusive attributes where we currently generate them.

[dougbu]

Decision is to remove the legacy (non-Unobtrusive) client-side validation feature from MVC 6.0. Current MVC 6.0 implementation is incomplete and our efforts are better spent on core scenarios. Similar feature in MVC 5.2 is likely very little-used since it relies on a JavaScript package that isn't even in NuGet.

Therefore the ask here is to remove ViewContext.UnobtrusiveJavaScriptEnabled and anything relying on that property.

[dougbu]

Fixed in commits b5dcc98 and 43d1253.