staging: unisys: get rid of serialloopbacktest

serialloopbacktest is no longer supported. Get rid of it.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>