Properly clear cookies when logging out.

When a user logs out, buddypress.js attempts to clear all cookies. However,
this now fails with BP 2.1.

The problem is when we enqueued our JS assets (#5208), we upgraded
jquery.cookie to 1.4.1. v1.4.1 uses a new routine to clear cookies.

This commit updates our clear cookies calls to this updated routine.

See https://buddypress.trac.wordpress.org/ticket/5909