Cleanup s4u2proxy in mag_auth_basic

It doesn't have any effect since we set GSS_C_DELEG_FLAG
when we initiate client credentials so we always get
delegated TGT regardless of constrained delegation.

This commit is not intended to change the current behaviour.

See #70

Reviewed-by: Simo Sorce <simo@redhat.com>
Closes #70
Closes #72