Remove the `.dat` file

I propose we remove this line: https://git.ligo.org/lscsoft/bilby/-/blob/master/bilby/core/sampler/dynesty.py#L622

Currently, the dynesty sampler dumps a rough set of samples on each checkpoint using rejection sampling. However, these:

1. Do not contain the complete set of posteriors (no reconstruction)
2. Differ from the final data product in a way we have not investigated

As such, they cause confusion for users (especially if the run fails, but they still see samples).

Any objections?