Remove proxy repository Mirrors feature

The "mirrors" tab in a proxy repository configuration was designed back when Maven Central was not on a CDN.

It works by retrieving artifacts and poms from the mirror, and metadata and checksums from the master. This is pointless now that central is on a CDN. There is only one mirror left (ibiblio) and nobody should be using that as a mirror.

At this point the feature is only causing confusion. I think we should remove the mirrors tab from the UI, and ignore any existing mirror configuration.