chore: drop the leftover root bunfig.toml

Nothing runs bun test from the root anymore, so its config is dead
weight.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GaLeuxVikb3iH2Etw2tHe7

Author: codebam