RDMA/hns: Remove unnecessary flush operation for workqueue

As a flush operation is implemented inside destroy_workqueue(), there is
no need to do flush operation before.

Fixes: bfcc681b ("IB/hns: Fix the bug when free mr")
Fixes: 0425e3e6

 ("RDMA/hns: Support flush cqe for hip08 in kernel space")
Link: https://lore.kernel.org/r/1618305087-30799-1-git-send-email-liweihang@huawei.com
Signed-off-by: Yixian Liu <liuyixian@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>