systemd: Remove WantedBy=sockets.service in [Install]

.socket systemd units already automatically get a
Before=sockets.service, and the udev rule we have will ensure that when
the appropriate virtio device is there, the socket unit will be started,
so this WantedBy dependency is not needed, and is in part responsible
for these 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>