iscsi-target: Remove left-over iscsi_target_do_login_io

There is no need for iscsi_target_do_login_io() anymore in modern code,
so go ahead and call iscsi_target_do_tx_login_io() directly within
iscsi_target_do_login().

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>