Validator interface

| Q             | A
| ------------- | ---
| Bug fix?      | sort of
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | 

- [Validator] fix return doc of ClassMetadata::getCascadingStrategy (int instead of bool)
- [Validator] remove duplicate interface implementations (they are already implemented by extension)