udev: don't let systemd-udev-settle delay basic.target

It is no longer possible to manually enable systemd-udev-settle.service,
so its only use is by legacy services explicitly pulling it in. It makes
sense for these services to also explicitly order themselves after
udev-settle.service, which makes After=basic.target redundant.

This should reduce the negative effect on boot-time of having to enable
legacy services such as lvm.service.