[bart] fix config.classif_dropout
This breaks backwards compatibility on saved `classif_dropout`, but from my checks this is always set to 0 (so incorrect) anyways and will stay 0, so I'm not too concerned.<|||||>```python
from transformers import BartConfig
config_to_save = BartConfig.from_pretrained('facebook/bart-base', classif_dropout=0.42)
config_to_save.classif_dropout # AttributeError
```