nvmet-fc: remove redundant del_work_active flag

The transport has a del_work_active flag to avoid duplicate scheduling
of the del_work item. This is redundant with the checks that
schedule_work() makes.

Remove the del_work_active flag.

Signed-off-by: James Smart <jsmart2021@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>