Groups Reaper functionality no longer deletes prometheus metrics

[DaveCLowe]

https://github.com/linkedin/Burrow/commit/718a013595569e9da196ebd4d09f7c4e7d27eeb9 commit reverted the change which deleted metrics from expired consumer groups.

Metrics are currently preserved and served up for expired consumer groups (those which are not listed under the /consumer route).

@bai I'm unsure why that change needed to be reverted?