chore: remove obsolete /api/modelstats endpoint (#2740)

This endpoint is only consumed by the legacy kilocode-legacy extension
(KiloModelInfoView.tsx). That code path wraps the fetch in try/catch,
reports the error via telemetry, and renders without pricing stats when
the request fails, so removal degrades gracefully.

Co-authored-by: kiloconnect[bot] <240665456+kiloconnect[bot]@users.noreply.github.com>

Author: kilo-code-bot[bot]