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

[via llvm-commits]

================
@@ -101,20 +101,6 @@ environment_definitions:
 
 
 steps:
-- label: Bootstrapping build
----------------
EricWF wrote:

Great!

Setting up the Mac OS X bots should be really easy too! Let me know if you want some help.

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