ci: remove aarch64-darwin-validate from fast-ci pipelines

We have much less darwin runners and they tend to be more flaky than other platforms, and even when they don't break, they can become a bottleneck for busy days when many MR pipelines are queued. Therefore, this patch removes aarch64-darwin-validate from the (default) fast-ci pipelines in order to make MR pipelines block for shorter time and improve contributor experience.

Marge pipelines and validate pipelines on master/ghc-x.y branches are unaffected. The only downstream jobs that use aarch64-darwin-validate are test-primops jobs and aarch64-darwin-validate is still run if an MR has test-primops label. And as usual, full-ci label can be used to force aarch64-darwin-validate to be run when it's needed.