This reverts commit ab6c8643d451b04884e938cd7ed2186663d32a96. Issue
https://github.com/NixOS/nix/issues/609 has been resolved, the new Nix
version is available after 86eaeb4c0a31e623c01f0d39fd5b3e64ce5f80b5, and
using nix-collect-garbage has the advantage that the '-d' flag is
available, which nix-store doesn't have.