iscsi-target: Drop legacy iscsi_target_tq.c logic

Now that iscsi_conn allocates new [rx,tx] threads using kthread.h
primitives on the fly, and kthread_stop() is called directly during
connection shutdown, it's time to go ahead and drop iscsi_target_tq.c
legacy code.

The use of multiple struct completion in iscsi_activate_thread_set()
has been proven to cause issues during repeated iser login/logout.

Tested-by: Sagi Grimberg <sagig@mellanox.com>
Cc: Slava Shwartsman <valyushash@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target.c

@@ -33,7 +33,6 @@
 #include <target/iscsi/iscsi_target_core.h>
 #include "iscsi_target_parameters.h"
 #include "iscsi_target_seq_pdu_list.h"
-#include "iscsi_target_tq.h"
 #include "iscsi_target_configfs.h"
 #include "iscsi_target_datain_values.h"
 #include "iscsi_target_erl0.h"
@@ -4360,8 +4359,6 @@ int iscsit_close_connection(
 
 	iscsit_put_transport(conn->conn_transport);
 
-	conn->thread_set = NULL;
-
 	pr_debug("Moving to TARG_CONN_STATE_FREE.\n");
 	conn->conn_state = TARG_CONN_STATE_FREE;
 	kfree(conn);