Merge registry instances and remove Instance interface

[stanislav-zeman]

Each service registry currently implements its own implementation of service instance. The instance implementations are currently all the same. I suggest merging all these implementations into a common instance type and removing the generic interface as it is not required and this need will most likely not change in the future.

The service type defined in the registry directory could also be removed but that needs further inspection.

Jira: VIDEO-1384