[red-knot] Remove global `dead_code` from red-knot-server (#17229)
## Summary
Use more local `expect(dead_code)` suppressions instead of a global
`allow(dead_code)` in `lib.rs`.
Remove some methods that are either easy to add later, are less likely
to be needed for red knot, or it's unclear if we'd add it the same way
as in ruff.
