test: remove unused empty fixture

The not-quite-empty directory called "empty" in fixtures was replaced in
tests with `tmpdir.path` (after calling `tmpdir.refresh()` to ensure
that it is empty). The fixture is removed in this commit.

PR-URL: nodejs#21044
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>