RFC 76 - Turn off the scheduler service

Comments: #76
Initially Proposed by: @djmitche

Summary

The scheduler service has been deprecated for a long time. Let's turn it off.

Details

The major known user of this service is release engineering (c.f. https://bugzilla.mozilla.org/show_bug.cgi?id=1259627) and it's unclear how difficult it would be to stop using the service.

Implementation

Tracker: https://bugzilla.mozilla.org/show_bug.cgi?id=1399437