[Validator] fix return doc of ClassMetadata::getCascadingStrategy
[Validator] remove duplicate interface implementations
minor #11706 [Validator] ClassMetadata doc fix (Tobion)

This PR was merged into the 2.5 branch.

Discussion
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[Validator] ClassMetadata doc fix

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| 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)

Commits
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1eec2d6 [Validator] remove duplicate interface implementations
6dd5592 [Validator] fix return doc of ClassMetadata::getCascadingStrategy