rds: ib: Remove unused extern definition

rds: ib: Remove unused extern definition

In the old days, RDS used FMR (Fast Memory Registration) to register
IB MRs to be used by RDMA. A newer and better verbs based
registration/de-registration method called FRWR (Fast Registration
Work Request) was added to RDS by commit 1659185fb4d0 ("RDS: IB:
Support Fastreg MR (FRMR) memory registration mode") in 2016.

Detection and enablement of FRWR was done in commit 2cb2912d6563
("RDS: IB: add Fastreg MR (FRMR) detection support"). But said commit
added an extern bool prefer_frmr, which was not used by said commit -
nor used by later commits. Hence, remove it.

Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Link: https://patch.msgid.link/20250905101958.4028647-1-haakon.bugge@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>