[Task] Remove KubeEnv discovery plugin

[andrewazores]

This internal "discovery plugin" isn't very helpful, since it uses k8s environment variables to detect target applications. This means that this discovery can only see target applications that were up before the Cryostat instance, and will never notice if those targets are modified or lost until the Cryostat instance is restarted. This is not a useful discovery mechanism, particularly now with how the discovery systems have evolved.