chore: remove checked-in openapi.json and update workflow (#181)

API reference docs are now sourced from the live
`app.kosli.com/api/v2/openapi.json` endpoint (configured in
`config/navigation.json`), so the checked-in spec and its update
workflow are no longer needed.

- Removed `api-reference/openapi.json`
- Removed `.mintlify/workflows/update-api-reference.md`

Co-authored-by: mintlify[bot] <109931778+mintlify[bot]@users.noreply.github.com>

Author: mintlify[bot]