Internal superclass still generated

[lars-reimann]

Describe the bug

Internal superclasses should

1. not be included in the parent type list of a class (sub),
2. not count towards the limit of one superclass per class,
3. push their own members to public subclasses (feat: Generate stubs for public methods of private classes which are superclasses of public classes #64).

Neither of these requirements seems to fully work yet, e.g. for the AdaBoostRegressor:

// TODO Safe-DS does not support multiple inheritance.
class AdaBoostRegressor(
    learner: Regressor? = null,
    @PythonName("maximum_number_of_learners") maximumNumberOfLearners: Int = 50,
    @PythonName("learning_rate") learningRate: Float = 1.0
) sub Regressor, _AdaBoostBase {
    /**
     * The base learner used for training the ensemble.
     */
    attr learner: Regressor?
}

1. _AdaBoostBase should not appear
2. The TODO should not appear
3. The members of _AdaBoostBase should appear in AdaBoostRegressor.

To Reproduce

Generate stubs for safe-ds.

Expected behavior

See description.

Screenshots (optional)

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Additional Context (optional)

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