scmutil: try-delete `.hg/store/requires` if store requirements are empty When downgrading from a shared-safe repository to non-shared-safe repository, we end up in a case where we had requirements stored in `.hg/store/requires` but no longer want them there. Let's explicitly try delete the `.hg/store/requires` file if store requirements are empty. Differential Revision: https://phab.mercurial-scm.org/D9357