[llvm] Revert "Partially Reapply "[libcxx] Move premerge jobs to k8s mode runner set (#221395)"" (PR #225896)

[Louis Dionne via llvm-commits]

ldionne wrote:

I opened https://github.com/llvm/llvm-project/issues/226230 to track fixing this, and to have a single place to put the various observations.

https://github.com/llvm/llvm-project/pull/225896