staging: Remove the drivers for the Unisys s-Par

The Unisys sub-tree of drivers/staging contains three drivers for the
"Unisys Secure Partition" (s-Par(R)): visorhba, visorinput, visornic.

They have no maintainers, in fact the only one that is listed in
MAINTAINERS has an unreacheable email address. During 2021 and 2022
several patches have been submitted to these drivers but nobody at
Unisys cared of reviewing the changes. Probably, also the
"sparmaintainer" internal list of unisys.com is not anymore read by
interested Unisys' engineers.

Therefore, remove the drivers/staging/unisys directory and delete the
relevant entries in the MAINTAINERS, Kconfig, Makefile files, then
remove also the drivers/visorbus directory which is not anymore needed
(it contained the driver for the virtualized bus for the Unisys s-Par
firmware).

Cc: David Kershner <david.kershner@unisys.com>
Cc: <sparmaintainer@unisys.com>
Cc: Ken Cox <jkc@redhat.com>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20220414103217.32058-1-fmdefrancesco@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>