Revert "nvmet-rdma: use a private workqueue for delete"

This reverts commit 2acf70ad

.

The commit never really fixed the intended issue and caused all
kinds of other issues, including a use before initialization.

Suggested-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>