Drop pci-stub from the load-modules service

pci-stub is available on Ubuntu from modules-extra kernel packages,
however, those are not always installed.

By the looks of it, this module is no longer supported by Libvirt either
(presumably in favor of VFIO):
b8e7e9be9a

Change-Id: I0db4d43d458893493232e150ae2f5b2bb7e05772
Closes-Bug: #1900113

snap-overlay/bin/load-modules

@@ -2,4 +2,4 @@
 
 set -ex
 
-modprobe -a vhost vhost-net vhost-scsi vhost-vsock pci-stub vfio nbd dm-mod dm-thin-pool dm-snapshot iscsi-tcp target-core-mod
+modprobe -a vhost vhost-net vhost-scsi vhost-vsock vfio nbd dm-mod dm-thin-pool dm-snapshot iscsi-tcp target-core-mod