Simplify db connection strategy

[MiskoG]

Created after the discussions in #499

We could do two things based on what you said (@nriss)

(1) Remove Pyrog/River's backend logic that deals with anything else than Postgres

(2) Think about a new UX workflow around projects and db connection, as there may be only one (moreover the same) db connection in every project
I don't know if we want to work on it right now as we're experimenting other stuff in parallel (DBT on FHIR), and even supposing that we are already using the new Pyrog, I don't see that issue as a major painpoint for the moment.

From my perspective
(1) is mainly backend work - it implies also minor work on the front, like removing dropdowns asking for DB type.
(2) is UX, frontend and backend.