tidyness check shouldn't happen on DBIX schema files

1. Edit Koha/Schema/Result/City.pm
2. Do any change in the customizable part at the end
3. git commit --no-verify (circumvent koha-testing-docker#478)
4. run qa script: qa
5. See that it complains about untidiness. Because the generated part of that file isn't tidy according to our rules. But it's managed and generated by dbic so it's not relevant to enforce the exact same formatting policy. And the few times we add customized code at the end don't warrant having formatting policy enforcement.

Koha/Schema.pm is also a DBIX schema file but it's at an unusual place.