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

[Mehdi Amini via llvm-commits]

https://github.com/joker-eph approved this pull request.

Do you still plan to bring this back later or do you want to invest in buildbot for the foreseeable future?
(Your RFC didn't seem like a complete dead end to me necessarily)

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