Remove `E999` to find diagnostic severity (#12080)
## Summary
This PR removes the need to check for `E999` code to find the diagnostic
severity in the server.
**Note:** This is just removing a redundant check because all
`ParseErrors` are converted to `Diagnostic` with default `Error`
severity by
https://github.com/astral-sh/ruff/blob/63c92586a10bfa9b75db9cb87a9ac08618a2ed95/crates/ruff_server/src/lint.rs#L309-L346
## Test Plan
Verify that syntax errors are still shown with error severity as it did
before:
<img width="1313" alt="Screenshot 2024-06-28 at 09 30 20"
src="https://github.com/astral-sh/ruff/assets/67177269/75e389a7-01ea-461c-86a2-0dfc244e515d">
