Handle 'drop' together with None to drop estimator in VotingClassifier/VotingRegressor

As mentioned in the following https://github.com/scikit-learn/scikit-learn/pull/11047#discussion_r264114338, the `VotingClassifier` and `VotingRegressor` should accept `'drop'` to drop an estimator from the ensemble is the same way that `None` is doing now.