Nightly builds on mozilla-esr31 will be shut off at the end of this week

Hi folks,

We've been running en-US Nightly builds on ESR branches since the
original ESR branch (mozilla-esr10). As far as I can tell, their only
reason for existing was because QE ran some update tests on them. QE is
no longer doing that, so unless someone has a strong reason for them
continuing to exist I'm going to shut them off at the end of this week.
These builds were never intended for users, and dead end whenever an ESR
branch is EOL'ed.

If you need non-release ESR builds for whatever reason you can find the
CI builds through https://treeherder.mozilla.org/#/jobs?repo=mozilla-esr31.

- Ben