Removes lingering kubelet plugins on snap removal (#727)

At the moment, the kubelet plugins in /var/lib/kubelet/plugins/ are not
removed, even though the Pods associated with those plugins have been
removed.

This means that on reinstallation, kubelet will try to access those
inexistent plugins through the *.sock files, including for plugins we
don't want anymore, and delaying registration for the ones we do want.

k8s/lib.sh

@@ -71,6 +71,11 @@ k8s::remove::containers() {
   # umount lingering volumes by force, to prevent potential volume leaks.
   cat /proc/mounts | grep /run/containerd/io.containerd. | cut -f2 -d' ' | xargs -r -t umount -f || true
   cat /proc/mounts | grep /var/lib/kubelet/pods | cut -f2 -d' ' | xargs -r -t umount -f || true
+
+  # remove kubelet plugin sockets, as we don't have the containers associated with them anymore,
+  # so kubelet won't try to access inexistent plugins on reinstallation.
+  find /var/lib/kubelet/plugins/ -name "*.sock" | xargs rm -f || true
+  rm /var/lib/kubelet/plugins_registry/*.sock || true
 }
 
 # Run a ctr command against the local containerd socket