Follow up to JsonResult changes

Changing the WebAPI shim to use the new and improved JsonResult.
> not sure I follow this change. Why do we have an obsolete class in the box?

We have a few obsolete classes in the shim specifically to show an error, and tell the use what the corresponding MVC type is when there's a direct replacement.

> And why remove the jsonresult from the shim

There was never a JsonResult in the shim. The plan was always to get users onto the new JsonResult. Due to the bug that I just fixed, we had to use ObjectResult directly in the shim.