v3: Remove unused mutex in RuleGroup

[nacx]

This just ports #381 to the v3/dev branch.

Thank you for contributing to Coraza WAF, your effort is greatly appreciated
Before submitting check if what you want to add to coraza list meets quality standards before sending pull request. Thanks!

Make sure that you've checked the boxes below before you submit PR:

• My code includes positive and negative tests.
• I have an appropriate description with correct grammar.
• I have read Contribution guidelines, maintainers note and Quality standard.
• My code is properly linted and passes pre-commit tests.

Thanks for your contribution ❤️