Kill $wgEchoCohortInterval

Authored by Legoktm.


Kill $wgEchoCohortInterval

The Echo cohort study was finished in July 2013 and hasn't been used
since. The code was also checking in reverse order, for example it
checked registration before checking whether the survey was still

EchoHooks::isEchoDisabled() was removed, and all callers were checked to
make sure they were also checking for anonymity.
EchoNotificationController::doNotification() will throw an exception if
the user is anonymous, since it shouldn't be possible to get an
anonymous user into that code path.

Bug: T101047
Change-Id: Iada2f6d2066c0f6bba5cc58aeb03d687632ac5a4