fix: Don't assign traintainers in tooling reviewers pool

What does this MR do and why?

This was brought up in Slack that reviewers suggestion for the :tooling category was mistakenly suggesting backend trainee maintainers: https://gitlab.slack.com/archives/CMA7DQJRX/p1658358754673849

I think this is misleading, and we should have enough reviewers for :tooling: now without needing to do that.

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