[docs] manual mailmap checking is no longer necessary

docs/RELEASE_CHECKLIST.md

@@ -25,6 +25,5 @@
  1. [ ] Add a new section in
         [`docker/docs/releases/Dockerfile`](https://github.com/pymor/docker/blob/main/docs/releases/Dockerfile).
  1. [ ] All developers check if (stale) branches can be pruned.
- 1. [ ] All developers check for mailmap correctness.
  1. [ ] Remove deprecated features in main.
  1. [ ] Close the [GitHub milestone](https://github.com/pymor/pymor/milestones) for the release.