chore: remove `mutation-testing` from `contrib`

[fdefelici]

Due to this PR #6137 and stacks-actions issue stacks-network/actions#80, the stacks-core/contrib/tools/local-mutation-testing.sh and related README can be removed

[github-actions]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.