`tldr --update_cache` does not delete local-only pages

How to reproduce on Linux:

1. Download all pages by running `tldr -u`
2. Go to `~/.cache/tldr/pages/linux/` dir
3. Create a test file: `cp cp.md zzzzz.md`
4. Running `tldr zzzzz` now shows the copied `cp` page
5. Run `tldr -u` again
6. See that the file `zzzzz.md` is still present.