* remove IPVS metrics

Remove metrics for IPVS services when the IPVS service is deleted so
that the number of metrics does not grow without bound.

Fixes #734

* delete metricsMap key when IPVS service is removed

Delete the key in NetworkServicesController.metricsMap when the
respective IPVS configuration is removed.

Remove a period from a comment to conform to kube-router norms

* cleanup stale metrics in a distinct method

* remove unnecessary error return value on cleanupStaleMetrics