[llvm] Remove fork handling from release issue workflow (PR #79310)

[Nikita Popov via llvm-commits]

nikic wrote:

The one remaining problem now is that apparently comments posted by the github-actions bot don't trigger new actions, so the `/branch` comment gets ignored if it's posted by the cherry-pick workflow. Ideally we would just directly create the pull request in the cherry-pick workflow -- I don't think there's a good reason to have the intermediate `/branch` comment at all.

https://github.com/llvm/llvm-project/pull/79310