systemd: Remove unneded virtio-port dependencies

The udev rule already adds SYSTEMD_WANTS=spice-vdagentd.socket
to the relevant virtio devices, which automatically adds Wants=${device}
to spice-vdagentd.socket (see 'systemctl show spice-vdagentd.socket').
Adding a Requisite/After for these ports is at best redundant, and most
likely wrong as this is causing boot time warnings:

[ INFO ] dev-virtio\x2dports-com.redhat.spice.0.device is not active.
[DEPEND] Dependency failed for Activation socket for spice guest agent daemon.

https://bugzilla.redhat.com/show_bug.cgi?id=1545212



Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>