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

[via llvm-commits]

EricWF wrote:

> hanks 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.

Sounds good. I'm working on moving the android builders too, and I'll be happy to help you move the mac builders too. 
It should be pretty simple.

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