chore(gh): remove automerge workflow

[jhchabran]

I noticed while QA'ing release stuff, that we have a workflow that runs https://github.com/pascalgn/automerge-action which is failing.

After closer inspection, this works by looking at the automerge label, which has just 3 PRs from two years ago.

Therefore, it's safe to simply drop it. And we weren't pinning the action sha anyway, so that's bad too.

Checklist

• Sister deploy-sourcegraph change:
• Sister customer-replica change (if necessary, for any changes affecting pure-docker or configuration):
• All images have a valid tag and SHA256 sum

Test plan

CI