RDMA/siw: Remove unused siw_mem_add

siw_mem_add() was added in 2019 by commit 2251334d ("rdma/siw:
application buffer management") but has remained unused.

Remove it.

Link: https://patch.msgid.link/r/20250505210226.88994-1-linux@treblig.org


Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>