[llvm] [Github][CI] Remove premerge container (PR #123483)

[Nathan Gauër via llvm-commits]

https://github.com/Keenuts approved this pull request.

Fine with the code change, but the PR description should reference an LLVM-zorg issue which explains the issues and switch, and this issue should link to the upstream konnectivity issue.
(Otherwise we will forget)

https://github.com/llvm/llvm-project/pull/123483