Remove 4-way cookie purge logic

Summary: HPHP has behaviorial differences from PHP which make this logic
problematic and we provide a good error message to users when there's a cookie
issue now, so unsplit the cookie logic and just clear the same cookie we'd
otherwise set, as per ssl / base domain.

Test Plan: Logged in and out of my local install.

Reviewers: jungejason

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 876