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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> So will this be the container used on all the runners?

All the Linux runners, yes. Originally we used one of the official Github Actions images that would then spawn a new container where all the work was actually performed. We had reliability issues with that, so we're going back to running the runner/jobs in a single container. We will try and switch back to the original technique when we can cause it adds some nice flexibility, but we need to get the reliability problems fixed first.

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