Since we changed the /api/v1/ routes to disallow session authentication
we also removed their reliance on CSRF. However, we left the
ReverseProxy authentication here - but this means that POSTs to the API
are no longer protected by CSRF.

Now, ReverseProxy authentication is a kind of session authentication,
and is therefore inconsistent with the removal of session from the API.

This PR proposes that we simply remove the ReverseProxy authentication
from the API and therefore users of the API must explicitly use tokens
or basic authentication.

Replace go-gitea#22077
Close go-gitea#22221 
Close go-gitea#22077 

Signed-off-by: Andrew Thornton <art27@cantab.net>