Remove per-diagnostic check for fixability

[charliermarsh]

Summary

Throughout the codebase, we have this pattern:

let mut diagnostic = ...
if checker.patch(Rule::UnusedVariable) {
    // Do the fix.
}
diagnostics.push(diagnostic)

This was helpful when we computed fixes lazily; however, we now compute fixes eagerly, and this is only used to ensure that we don't generate fixes for rules marked as unfixable.

We often forget to add this, and it leads to bugs in enforcing --unfixable.

This PR instead removes all of these checks, moving the responsibility of enforcing --unfixable up to check_path. This is similar to how @zanieb handled the --extend-unsafe logic: we post-process the diagnostics to remove any fixes that should be ignored.

[charliermarsh]

This is the actual code change.

[charliermarsh]

This is a five line change, which then requires removing those if guards in nearly every rule.

[github-actions]

PR Check Results

Ecosystem

✅ ecosystem check detected no changes.