Fix "method_complexity" issue in src/app/modules/common/validators/cpf/cpf.validator.ts

[Onnion]

Function checkCpf has a Cognitive Complexity of 8 (exceeds 5 allowed). Consider refactoring.

https://codeclimate.com/github/Onnion/angular-crud/src/app/modules/common/validators/cpf/cpf.validator.ts#issue_5d44719ec923180001000048