[BartTokenizer] remove inheritance on RobertaTokenizer
# What does this PR do?

Continue making tokenizers independent. This PR refactors the `BartTokenizer` to remove dependency on `RobertaTokenizer`
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for tackling this! Should we maybe also add some `# Copied from ...` statements here to make the code more robust to changes?