IB/mlx4: Remove set but not used variable 'pd'

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

drivers/infiniband/hw/mlx4/qp.c: In function '_mlx4_ib_destroy_qp':
drivers/infiniband/hw/mlx4/qp.c:1612:22: warning:
 variable 'pd' set but not used [-Wunused-but-set-variable]

Fixes: e00b64f7 ("RDMA: Cleanup undesired pd->uobject usage")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>