Fix an issue with clearing cookies when a cookie prefix is set

Summary: If a cookie prefix is set (as on the Phacility cluster), we end up double-namespacing cookies when trying to remove them. This can make logging out produce a cookie error.

Test Plan: Logged out locally with cookie prefix, got normal logout workflow.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11282