chore: Remove tbx package (#13133)
## Why
`tbx` is no longer needed in this repository. Removing it avoids
carrying unused sandbox tooling and extra workspace dependencies.
## What
Removes the `@turbo/tbx` workspace package, its root script entry, and
the now-unused lockfile entries.
## How
Ran `pnpm install --lockfile-only` after deleting the package so the
lockfile reflects the remaining workspace graph. Pre-push hooks passed,
including formatting, TOML check, Rust format, lint, and workspace
check.
