This patch drops legacy active_ts_list usage within iscsi_target_tq.c
code.  It was originally used to track the active thread sets during
iscsi-target shutdown, and is no longer used by modern upstream code.

Two people have reported list corruption using traditional iscsi-target
and iser-target with the following backtrace, that appears to be related
to iscsi_thread_set->ts_list being used across both active_ts_list and
inactive_ts_list.

[   60.782534] ------------[ cut here ]------------
[   60.782543] WARNING: CPU: 0 PID: 9430 at lib/list_debug.c:53 __list_del_entry+0x63/0xd0()
[   60.782545] list_del corruption, ffff88045b00d180->next is LIST_POISON1 (dead000000100100)
[   60.782546] Modules linked in: ib_srpt tcm_qla2xxx qla2xxx tcm_loop tcm_fc libfc scsi_transport_fc scsi_tgt ib_isert rdma_cm iw_cm ib_addr iscsi_target_mod target_core_pscsi target_core_file target_core_iblock target_core_mod configfs ebtable_nat ebtables ipt_MASQUERADE iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 ipt_REJECT xt_CHECKSUM iptable_mangle iptable_filter ip_tables bridge stp llc autofs4 sunrpc ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 xt_state nf_conntrack ip6table_filter ip6_tables ipv6 ib_ipoib ib_cm ib_uverbs ib_umad mlx4_en mlx4_ib ib_sa ib_mad ib_core mlx4_core dm_mirror dm_region_hash dm_log dm_mod vhost_net macvtap macvlan vhost tun kvm_intel kvm uinput iTCO_wdt iTCO_vendor_support microcode serio_raw pcspkr sb_edac edac_core sg i2c_i801 lpc_ich mfd_core mtip32xx igb i2c_algo_bit i2c_core ptp pps_core ioatdma dca wmi ext3(F) jbd(F) mbcache(F) sd_mod(F) crc_t10dif(F) crct10dif_common(F) ahci(F) libahci(F) isci(F) libsas(F) scsi_transport_sas(F) [last unloaded: speedstep_lib]
[   60.782597] CPU: 0 PID: 9430 Comm: iscsi_ttx Tainted: GF 3.12.19+ #2
[   60.782598] Hardware name: Supermicro X9DRX+-F/X9DRX+-F, BIOS 3.00 07/09/2013
[   60.782599]  0000000000000035 ffff88044de31d08 ffffffff81553ae7 0000000000000035
[   60.782602]  ffff88044de31d58 ffff88044de31d48 ffffffff8104d1cc 0000000000000002
[   60.782605]  ffff88045b00d180 ffff88045b00d0c0 ffff88045b00d0c0 ffff88044de31e58
[   60.782607] Call Trace:
[   60.782611]  [<ffffffff81553ae7>] dump_stack+0x49/0x62
[   60.782615]  [<ffffffff8104d1cc>] warn_slowpath_common+0x8c/0xc0
[   60.782618]  [<ffffffff8104d2b6>] warn_slowpath_fmt+0x46/0x50
[   60.782620]  [<ffffffff81280933>] __list_del_entry+0x63/0xd0
[   60.782622]  [<ffffffff812809b1>] list_del+0x11/0x40
[   60.782630]  [<ffffffffa06e7cf9>] iscsi_del_ts_from_active_list+0x29/0x50 [iscsi_target_mod]
[   60.782635]  [<ffffffffa06e87b1>] iscsi_tx_thread_pre_handler+0xa1/0x180 [iscsi_target_mod]
[   60.782642]  [<ffffffffa06fb9ae>] iscsi_target_tx_thread+0x4e/0x220 [iscsi_target_mod]
[   60.782647]  [<ffffffffa06fb960>] ? iscsit_handle_snack+0x190/0x190 [iscsi_target_mod]
[   60.782652]  [<ffffffffa06fb960>] ? iscsit_handle_snack+0x190/0x190 [iscsi_target_mod]
[   60.782655]  [<ffffffff8106f99e>] kthread+0xce/0xe0
[   60.782657]  [<ffffffff8106f8d0>] ? kthread_freezable_should_stop+0x70/0x70
[   60.782660]  [<ffffffff8156026c>] ret_from_fork+0x7c/0xb0
[   60.782662]  [<ffffffff8106f8d0>] ? kthread_freezable_should_stop+0x70/0x70
[   60.782663] ---[ end trace 9662f4a661d33965 ]---

Since this code is no longer used, go ahead and drop the problematic usage
all-together.
Reported-by: Gavin Guo <gavin.guo@canonical.com>
Reported-by: Moussa Ba <moussaba@micron.com>
Cc: stable@vger.kernel.org # 3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>