disable PackIt integration on the GitHub side

Summary:
We don't need this anymore, as we're using `pull_from_upstream` on the Fedora
side instead to make sure each weekly release is buildable

New config: https://src.fedoraproject.org/rpms/folly/blob/rawhide/f/packit.yaml

Reviewed By: davide125

Differential Revision: D54958528

fbshipit-source-id: 76e9730308731b1921f333aa2e401397cff079d2