Remove `start_height` input from `bitcoind_rpc::Emitter::new`.

**Describe the enhancement** We can just use `CheckPoint::floor_at` to start at a lower height, making the `start_height` parameter irrelevant. **Use case** Simplifies API. **Impact** - [ ] Blocking production usage - [x] Nice-to-have / UX improvement - [x] Developer experience / maintainability **Which backend(s) are relevant (if any)?** - [x] Bitcoin Core RPC