Delete CASGatewayCookie after successful authentication.

This allows people to reauthenticate after their mod_auth_cas session times out when using gateway mode.
I don't think I can do that because the bug and fix are in different invocations of mod_auth_cas.  Here's the scenario:

First visit: user arrives; mod_auth_cas set the CASGatewayCookie and redirects the user to the CAS server.

Second visit: user returns from the CAS server;  mod_auth_cas creates a session and sets CASCookie or CASSecureCookie.

Third and subsequent visits: user has a valid mod_auth_cas session.

Nth visit: user returns after session expires; mod_auth_cas should redirect user to CAS server to authenticate, but doesn't because the CASGatewayCookie still exists.

The fix deletes the CASGatewayCookie during the second visit.