[#81813770] Remove the Tahi Auth Check header

* References: https://www.pivotaltracker.com/story/show/81813770

--- FF & CT
Perhaps it's better to have the authorization checks always return a 200 OK response. Whether the user has access to the resource being checked could be sent in the response body.

This makes sense, since the authorization check is successful regardless of whether the user has access or not.

(This comment was co-authored by @faun.)