shell-completion: zsh: fix incorrect unescaping

shell-completion: zsh: fix incorrect unescaping

Previously the `_filter_units_by_property` completion function
outputs with a [zsh parameter expansion flag] `g:o:`. This means
that the returned result is unescaped as the zsh builtin `echo`,
except that octal escapes don’t take a leading zero. This seemed to
have worked back in the days when it was first introduced:

  6c9414a700a040be1d3160bd2336baac58a1da3e

But it now leads to incorrect over-unescaping; for example,

  system-systemd\\x2djournald.slice (correct)

is incorrectly completed by zsh in commands such as
`systemctl kill`:

  system-systemd-journald.slice (incorrect)

This commit fixes such problems by removing the `g:o:` flag.

See:
[zsh parameter expansion flag]: https://zsh.sourceforge.io/Doc/Release/Expansion.html#Parameter-Expansion-Flags