SPEC: Drop old OS conditions from spec file.
    
    It can be possible to build current master without samba
    on rhel5, but the spec file would be very complicated.
    It is better to simplify spec file.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/1974
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>