nubus: Make struct nubus_driver::remove return void

The nubus core ignores the return value of the remove callback (in
nubus_device_remove()) and all implementers return 0 anyway.

So make it impossible for future drivers to return an unused error code
by changing the remove prototype to return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Finn Thain <fthain@linux-m68k.org>
Link: https://lore.kernel.org/r/20210727080840.3550927-3-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>