[Github][CI] Stop running premerge checks on main

This patch stops running the premerge checks on the main branch. This is
based on some feedback in
https://discourse.llvm.org/t/rfc-running-premerge-postcommit-through-github-actions/86124.
We want to use buildbot for post commit testing. This should be covered
in the mean time by the current pemerge-monolithic-* bots even though
the configurations are not exactly the same. This also adds a bit of
capacity back to the cluster (although might not in the end when we
start running more substantial postcommit testing through buildbot).

This is primarily in preparation for turning the new premerge system on
as canonical. Without this, we run into warning fatigue issues as
described in the RFC above.

Reviewers: Keenuts, joker-eph, dschuff, cmtice, gburgessiv, lnihlen, tstellar

Reviewed By: Keenuts, joker-eph

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

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