Remove auto-merge GitHub actions

[RCheesley]

Early on we used an auto-merge job which would push approved PRs directly to the development branch, but we aren't using that any more and we don't want to do auto-merging because we need to ensure the correct branch is used and that any related PRs are merged first in mautic/mautic, for example.

So I think we should probably look to remove the workflow files and just manually approve when we're ready.