Solr publishes environment variables via the Metrics API

Much like sysPros, Solr apparently has published envVars through the metrics API since 9.0.

As I mentioned in SOLR-15019, this is a big security issue and it should be removed. Before the release of 9.0, the use of this within the PlacementPlugins was removed, but the real issue of publishing via the metrics API was never addressed. (Weird, because I remember testing this out...)

This is a security risk, because we have very little way of controlling what Environment Variables users use on their machines, and its too big of a burden to have them keep a list of these in their Solr.xml.

We should remove this "metric" and create a bug-fix release.