Remove GitLab performance regression CI

[masterleinad]

It seems that we won't be able to get the performance regression CI back to work in the same way we used it before ECP ended.

[aprokop]

Do we want to remove it? I disabled the actions, so it should not trigger. I don't see a pressing need for removal, though I understand the desire.

[masterleinad]

Do we want to remove it? I disabled the actions, so it should not trigger. I don't see a pressing need for removal, though I understand the desire.

It's just dead code at this point and having it makes the impression that we are using it while we don't. I also still get notifications from forks, see, e.g. https://github.com/dalg24/ArborX/actions/runs/7475756660.