net/rds: remove unused struct 'rds_ib_dereg_odp_mr'

net/rds: remove unused struct 'rds_ib_dereg_odp_mr'

'rds_ib_dereg_odp_mr' has been unused since the original
commit 2eafa1746f17 ("net/rds: Handle ODP mr
registration/unregistration").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Link: https://patch.msgid.link/20240930134358.48647-1-linux@treblig.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>