SPEC: Drop conditional build for krb5_local_auth_plugin
    
    It was mainly aimed for time when stable CentOS and
    rhel nightly had different versions of krb5.
    
    Anyway, rhel7.0 and rhel <= 6.6 are already out of support
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>