nvmet-loop: remove unused 'target_ctrl' in nvme_loop_ctrl

This field is never used since the introduction of nvme loopback by
commit 3a85a5de

 ("nvme-loop: add a NVMe loopback host driver").

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>