feat: add erase method to PluginSvc Registry class

This is required for FunctorFactory in LHCb's REC project.

@clemenci this relates to the discussion we had about the FunctorFactory. If I can use this to remove the "old" functors that are created when an old functor cache is loaded, I can then create new entries for the new functors that were just in time compiled.

Let me know what you think :)