simplify lemma lebesgue_measure_rat

[affeldt-aist]

Motivation for this change

The goal of this PR is to extract a lemma saying that a countable set has measure 0.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

- [ ] added corresponding documentation in the headers

Reference: How to document

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• PRs with disorganized commits are very likely to be squash-rebased.

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[affeldt-aist]

I will maybe merge if the CI is green since this is a small but clear improvement.
(Maybe there is a better name for the extracted lemma...)