Don't clear BackURL after MemberAuthenticator::authenticate()

It breaks logic flow, e.g. when

Its called by BasicAuth:requireLogin() when basic auth is enabled,
before any controller logic kicks in (on every HTTP request).
This means you can't use session-based BackURLs with basic auth enabled,
breaking flows like redirection after Facebook logins.

I can't see why a clear() was necessary here, looks like a overly
cautious way to prevent infinite loops? Can't see how those
would be caused by requireLogin() though.

Been there since all the way back in 2007: a377a67e540a33941e4a5436d169e4cfa93af58a
Merge pull request #3433 from chillu/pulls/authenticate-backurl-master

Don't clear BackURL after MemberAuthenticator::authenticate()