Drop pr-branch-auto-delete feature

[FloEdelmann]

It was implemented by GitHub: https://github.blog/changelog/2019-07-31-automatically-delete-head-branches-of-pull-requests/

On a repository's settings page, there is now a checkbox to enable the feature:

[fregante]

Lol that was fast!

cc @sindresorhus

[fregante]

This is good, but their feature has a few big disadvantages:

• it has to be enabled in each repo: our feature works everywhere.
• it has to be enabled by the owner: if the owner doesn't enable it, collaborators won't benefit from it.

For this, I think we should still keep it.

I wish GitHub at least enabled it by default in forks, where our feature doesn't apply.

[sindresorhus]

I agree, we should keep the feature. I have no intention of manually enabling that setting on 1000+ repos.