Remove "release:cherry-pick-failed" label if later cherry pick succeeds?

[nikic]

If the first cherry-pick fails but a later one succeeds, it would probably make sense to automatically drop the label, to make it clear that the backport request is not blocked on conflict resolution anymore.

cc @tstellar

[llvmbot]

@llvm/issue-subscribers-infrastructure

[nikic]

For reference, there's a patch for this here: https://reviews.llvm.org/D121032

[nikic]

Patch landed in 3929f91.