Don't show "you are already signed in" message on registration route

What does this MR do and why?

Removes the "You are already signed in." message when visiting the registration path and being redirected back.

#322929 (closed)

How to set up and validate locally

Sign in to the local GitLab, and visit http://localhost:3000/users/sign_up, you should not be presented with a banner that says "you are already signed in".

#398228 (closed)