[llvm] [Github][CI] Stop running premerge checks on main (PR #139358)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Do you still plan to bring this back later or do you want to invest in buildbot for the foreseeable future?

If Github gives us more control over notifications, I think we can revisit. Otherwise, I think the plan is to focus on getting builtbot setup (properly) for postcommit testing.

https://github.com/llvm/llvm-project/pull/139358