systemd-network-after-root-writable: Remove

This package provides the following unit:

    # /etc/systemd/system/network-after-root-writable.target
    [Unit]
    Description=network after root writable
    After=remount-rootfs.service udev-settle.service
    Before=network.service

    [Install]
    WantedBy=network.target

The systemd-provided services referenced as After depdencies have been
renamed:

- remount-rootfs.service -> systemd-remount-fs.service
- udev-settle.service -> systemd-udev-settle.service, udev-settled.target

When these services no longer exist, this unit doesn't have any
function, so a removal shouldn't break anything.

Instead of fixing it here, I'll try to solve existing race problems in
mxtools.