Target page entries used to exist for each user
that was notified for an event. They were
removed as the notifications were marked as read.
Now they remain so that the association between
pages and events can be used for moderation
regardless of the notifications read status.
This patch removes everything about
echo_target_page.etp_user from sql and php code.
Bug: T143959
Change-Id: Ib57510e6b0e9202a7e035f8ea59955dca8a0b24a
