switch-to-configuration-ng: fix dropins_removed comments

                // When the unit is disabled, the override files will be a symlink to /dev/null instead.

                dropins_removed =

                    // True if either no existing files or no disabled files (unit existed)

                    // True iff at least one current drop-in is not /dev/null,

                    // i.e. the instance was NixOS-managed in the old generation.

                    !current_dropins.all(is_unit_disabled)

                    // True if either no new files or all disabled new files (unit gone)

                    // True iff there are no new drop-ins, or all of them are

                    // /dev/null, i.e. the instance is gone in the new generation.

                    && new_dropins.all(is_unit_disabled);

            }

        }