Remove per-diagnostic check for fixability (#7919)

## Summary

Throughout the codebase, we have this pattern:

```rust
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.

crates/ruff_linter/src/linter.rs

@@ -260,6 +260,13 @@ pub fn check_path(
         }
     }
 
+    // Remove fixes for any rules marked as unfixable.
+    for diagnostic in &mut diagnostics {
+        if !settings.rules.should_fix(diagnostic.kind.rule()) {
+            diagnostic.fix = None;
+        }
+    }
+
     // Update fix applicability to account for overrides
     if !settings.extend_safe_fixes.is_empty() || !settings.extend_unsafe_fixes.is_empty() {
         for diagnostic in &mut diagnostics {