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.
