celery: drop config settings not supported in Celery 4

CELERY_SEND_TASK_ERROR_EMAILS has been removed, according to
https://docs.celeryproject.org/en/3.0/whatsnew-4.0.html#features-removed-for-simplicity :
"Tasks no longer sends error emails. This also removes support for
app.mail_admins, and any functionality related to sending emails."

The "ADMINS" prefix might be a left-over from the long removed
celery.conf.ADMINS - last seen on
https://docs.celeryproject.org/en/2.1-archived/reference/celery.conf.html#celery.conf.ADMINS .

The "ROUTES" prefix might refer to "CELERY_ROUTES" ... but it doesn't take a
simple string list anyway, so there is no point in treating it as a list value.