remote files completion: remove double-escaping

[fugidev]

Removes the double escaping in the remote files completion. This affects rsync and scp. Instead of for example a space character in a remote filename turning into \\\ , it will now correctly turn into \ . While scp apparently works with either one, rsync supports only the latter since version 3.2.4. This has been a problem for almost two years now.

For more context, see the corresponding issue from fish-shell: https://github.com/fish-shell/fish-shell/issues/8895