scsi: libiscsi: Remove iscsi_destroy_session

iscsi_session_teardown was the only user of this function. Function
currently is just short for iscsi_remove_session + iscsi_free_session.

Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
Acked-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>