[libcxx] [llvm] remove linux buildkite builders entirely (PR #73825)

[Louis Dionne via llvm-commits]

https://github.com/ldionne commented:

Thanks a lot for the cleanup, this was on my list! This should allow dropping the GCE runners on Linux altogether, right? Might be worth recording that benefit in the commit message.

https://github.com/llvm/llvm-project/pull/73825