Remove priority labels

https://about.gitlab.com/handbook/engineering/infrastructure/team/scalability/#priority-labels defines the Scalability::P[1-4] labels.

In practice, we pretty much only use ~"Scalability::P4": https://gitlab.com/gitlab-com/gl-infra/scalability/-/boards/2371067

This may be right, but I find it quite confusing when triaging the board. Does ~"Scalability::P4" mean it's actually nice-to-have, or does it just mean that it's not had an explicit priority set and so could be anything?

As it seems people aren't really using these much, I'm proposing we get rid of them. I'd also accept using them more often 🙂