ci: Remove GitLab UI WG from reviewer's roulette

During a recent WG meeting, we have discussed the gitlab-ui WG member approval rule.

This approval rule was initially created with a focus on our components styling guidelines that were still pretty new back then. Time has passed and it seems like those styling guidelines are being applied pretty consistently, without the intervention of WG members. Thus, the approval rule is becoming a bottleneck for new merge requests and it might be preferable to remove it altogether.

This MR removes the WG from the reviewer's roulette.

Let's discuss this change here. If we all agree to move forward with this, we will need to change the project's settings to also remove the approval rule itself.