winery: remove benchmark tooling

The stats are only used in the benchmark process and will not be exploited again.

We don't expect to be using Grid'5000 again so the benchmark code has not been maintained with expectations that it would work again. This removal allows us to streamline future refactorings as well.

A future objstorage benchmark tool could take inspiration from a lot of this code, but be agnostic to the backend used.