[TAPEX] Update drop_rows_to_fit
# What does this PR do?

This PR makes `drop_rows_to_fit` an attribute of `TapexTokenizer`, rather than a standalone `TruncationStrategy`.

The truncation strategies that can be used are the same as those of BART (as TAPEX is a BART model), meaning `truncation=True` will truncate to the maximum length. However, one can still randomly drop rows based on answers using this attribute.
_The documentation is not available anymore as the PR was closed or merged._<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Closing this as "drop_rows_to_fit" probably deserves to be a truncation strategy on its own.