Update to use datasets remove_cloumns method
# What does this PR do?

This PR updates the command used in the `Trainer` to drop the columns not used by the model to take advantage of the latest (well not so latest since it landed in datasets 1.4.0) `remove_columns` method. This adds the advantage of not modifying in place the dataset (as was done before) so the user does not have unexpected changes in their original datasets.

In consequence, the little hack needed in the question answering examples is now unnecessary.