Remove the deprecated E999 rule code

[MichaReiser]

Summary

The E999 rule code was deprecated as part of Ruff 0.5.

This PR now removes it entirely. Using E999 is now a hard error.

Test Plan

cargo test

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[AlexWaygood]

Overall this LGTM. The ecosystem output is noisy because you changed the target branch after filing the PR, but it doesn't look like there's anything E999-related in there!

I think there's still two references to E999 elsewhere in the repo that we should get rid of:

crates/ruff/tests/integration_test.rs:    // Select any rule except for `E999`, syntax error should still be shown.
crates/ruff_workspace/src/configuration.rs:                if matches!(rule.as_str(), "E999") {