netns: fix error when stopping netns

A short time after `netns delete` finishes, the peer link in the main
netns is automatically removed.
When `link del` is run before that, it fails with
`Cannot find device "nb-veth-br-*"` and the netns service enters a failed state.

modules/netns-isolation.nix

@@ -175,7 +175,6 @@ in {
           '') v.availableNetns;
           preStop = ''
             ${ip} netns delete ${netnsName}
-            ${ip} link del ${peer}
           '';
           serviceConfig = {
             Type = "oneshot";