This removes most of the code used for polylith/API mode.
This removes the `/api` internal endpoints entirely.
Binary size change roughly 5%:
```
51437560 Feb 13 10:15 dendrite-monolith-server # old
48759008 Feb 13 10:15 dendrite-monolith-server # new
```
This PR
- adds several tests for the clientapi, mostly around `/register` and
auth fallback.
- removes the now deprecated `homeserver` field from responses to
`/register` and `/login`
- slightly refactors auth fallback handling
