zsh: _systemctl: do not attempt to use "--system" by default (#3951)

zsh: _systemctl: do not attempt to use "--system" by default (#3951)

In 68c4f6d the following was added:

    local -a _modes; _modes=("--user" "--system")
    local _sys_service_mgr=${${words:*_modes}[(R)(${(j.|.)_modes})]:---system}

With the following comment:

> If neither are on the line, --system is set; for system services to be
> completed.

But it does not work as documented:

    % _modes=(--user --system)
    % words=()
    % echo ${${words:*_modes}[(R)(${(j.|.)_modes})]:---system}

However, it should not use `--system` in that case anyway, so this patch
removes the part that should cause a default to be used and adds some
comments.