Remove `/metrics`

[link2xt]

This is a follow-up to #328

We now have mtail metrics which are more flexible and can also count new registrations and similar metrics.

/metrics is created by a cronjob and is not authenticated, so we cannot expose much there and "pushing" metrics is not really compatible with prometheus model where it expects to pull metrics at any time and timestamps the data with the current timestamp.

For metrics like disk space admins can run node_exporter.