Remove unused mutex in RuleGroup

[nacx]

The RuleGroup documentation says it is not concurrent safe, but it still has a RWMutex that is not properly used: the GetRules method locks for read but none of the write methods honor the mutex, so the lock is not effective and the method is not concurrent safe (as opposed to what its doc string says).

Should we just remove the lock and keep it concurrent unsafe as the RuleGroup type documentation says, and remove the sync overhead, or should I better make the RuleGroup concurrent safe and properly lock on writes?

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[jptosso]

This is not used, and I think it should enhance performance.

Thank you for your contribution

[jcchavezs]

LGTM, would you port the change to v3/dev too?

[nacx]

Sure: #384