Branch protection restrictions not cleared

[Mark Rousskov]

It looks like we don't clear the branch protection user overrides, which means that we end up with an empty set (modulo maintain/admin permissions) of folks who can merge PRs.

https://github.com/rust-lang/sync-team/blob/master/src/github/mod.rs#L471 should probably be set to null if the users list is empty.

cc @rylev