RDMA/iwpm: Remove set but not used variable 'msg_seq'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/infiniband/core/iwpm_util.c: In function 'iwpm_send_hello':
drivers/infiniband/core/iwpm_util.c:811:6: warning:
 variable 'msg_seq' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit b0bad9ad

 ("RDMA/IWPM:
Support no port mapping requirements")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>