Unfortunately Hadoop tries to be too-clever and permanently keeps track of all names registered
so far as a Source, thus preventing further re-registration of the source with the same name.
In case of dynamic metrics tied to region-lifecycles, this becomes a problem because we would
like to be able to re-register and remove with the same name. Otherwise, it is resource leak.
This ugly code manually removes the name from the UniqueNames map.
TODO: May not be needed for Hadoop versions after YARN-5190.