target: Remove command flag CMD_T_BUSY

The patch that reworks task management function handling guarantees
that target_remove_from_state_list() is always called with CMD_T_BUSY
cleared. Since that function is the only function that tests that flag
this means that that flag is now superfluous. Hence remove that flag.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Andy Grover <agrover@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>