RDS: IB: Remove unused declarations

Commit f4f943c9 ("RDS: IB: ack more receive completions to improve performance")
removed rds_ib_recv_tasklet_fn() implementation but not the declaration.
And commit ec16227e ("RDS/IB: Infiniband transport") declared but never implemented
other functions.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Link: https://patch.msgid.link/20240731063630.3592046-1-yuehaibing@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>