It looks like we stopped using the sentry table in #9
and removed in #211.  The other functions were used for
other "checks", and also removed in #211.

This project shouldn't write anything to the DB anyway.

…ead of the `mirror_mirrors` database table

In practice, that's what we are already doing because these two env vars
are already set. This patch makes it more intentional and explicit,
while getting rid of yet another dependency to an unused database table.