rapidio: remove unused functions

rio_request_dma() and rio_dma_prep_slave_sg() were added in 2012 by commit
e42d98eb ("rapidio: add DMA engine support for RIO data transfers")
but never used.

rio_find_mport() last use was removed in 2013 by commit 9edbc30b
("rapidio: update enumerator registration mechanism")

rio_unregister_scan() was added in 2013 by commit a11650e1 ("rapidio:
make enumeration/discovery configurable") but never used.

Remove them.

Link: https://lkml.kernel.org/r/20250419203012.429787-3-linux@treblig.org


Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Cc: Alexandre Bounine <alex.bou9@gmail.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>