Remove unused `classifier_dropout` in configs
# What does this PR do?

Similar to #20554, but this time for `classifier_dropout`.
The existing checkpoints with this attribute in their config files could still be loaded via the `**kwargs` --> so won't fail.


@sgugger If you would prefer me to cleanup multiple different unused config attributes in a single PR, let me know 😉 


_The documentation is not available anymore as the PR was closed or merged._