Remove `id` and `useGet` from `ITerminologyService`

[ewoutkramer]

After working with ITS for a while, I now realize that useGet and id are only meaningful in a REST context:

• useGet: this distinction only exists at the HTTP level, not at the more conceptual ITS level.
• id: distinguishes between "instance" and "type" operation on the REST interface, but at the ITS level, this distinction should be gone (and turned into either a url or included VS/CodeSystem parameter), as the ITS implementations should conceptually not have to know about REST ids (these are mapped to actual resources/canonicals by the server endpoint).

I think both can be removed without making a breaking change using default implementations, overloads and [Obsolete].

[ewoutkramer]

@alexzautke - this is not a high prio, but as we're completely "into" TS at the moment, it would take less effort now to do than later. But I leave this at your discretion.