Remove duplicate namespace filter validation

What does this MR do and why?

AuditEvents::Group::ExternalStreamingDestination includes the AuditEvents::ExternallyStreamable concern, then re-declares three things the concern already provides word for word: the MAXIMUM_NAMESPACE_FILTER_COUNT constant, the validate :no_more_than_5_namespace_filters? registration, and the method behind it. This MR deletes all three.

The sibling AuditEvents::Instance::ExternalStreamingDestination relies on the concern alone, so the two classes now have the same shape.

Behaviour is unchanged. Nothing outside the class referenced the shadowed constant, and Ruby resolves Klass::CONST through included modules, so such a reference would keep working. The group spec already asserts a single error message for the over-limit case and runs the concern shared example.

References

None.

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