Ability to remove labels reported by clients

Currently, client reported labels remain associated with the client even if a new repack/build is deployed to the client with a different 'baked' in label. It would be beneficial to provide a means to remove or update labels reported from clients, either through some logic or by adopting new labels on client/repack update install and dropping the old.

A use-case for this functional is to allow for tracking custom client builds or repacks via labels, updating revisions as new client builds/repacks are deployed (while under the same GRR version).