target: Drop legacy se_cmd->check_release bit

Now with iscsi-target using modern se_cmd->cmd_kref accounting in
v3.10 code, it's safe to go ahead and drop the legacy release
codepath + se_cmd->check_release bit in transport_release_cmd()

Cc: Christoph Hellwig <hch@lst.de>
Cc: Roland Dreier <roland@kernel.org>
Cc: Kent Overstreet <koverstreet@google.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Cc: Moussa Ba <moussaba@micron.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

include/target/target_core_base.h

@@ -420,8 +420,6 @@ struct se_cmd {
 	int			sam_task_attr;
 	/* Transport protocol dependent state, see transport_state_table */
 	enum transport_state_table t_state;
-	/* Used to signal cmd->se_tfo->check_release_cmd() usage per cmd */
-	unsigned		check_release:1;
 	unsigned		cmd_wait_set:1;
 	unsigned		unknown_data_length:1;
 	/* See se_cmd_flags_table */