Allow users to remove machinesets by setting them to `null`

[simu]

Checklist

• [x] The PR has a meaningful title. It will be used to auto generate the changelog. The PR has a meaningful description that sums up the change. It will be linked in the changelog.
• [x] PR contains a single logical change (to build a better changelog).
• [ ] Update the documentation.
• [x] Categorize the PR by adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog.
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