tools: notifier: remove sql link between notification_subscribers and users.

This is done in preperation to snmp notifications which have no
connections to engine users. This includes removing subscriber_id
from event_notification_hist and not using a users email instead of
a notification_subscriber email.

Change-Id: Ibb91d064c5ecec5b335c32636ac432299c5f96f6
Signed-off-by: Mooli Tayer <mtayer@redhat.com>