netns: fix error msg when starting netns

Previously, the failing initial `netns delete` resulted in a
"Cannot remove namespace file ..." error visible in the journal
and status output.

modules/netns-isolation.nix

@@ -151,6 +151,7 @@ in {
           requiredBy = bindsTo;
           before = bindsTo;
           script = ''
+            ${ip} netns delete ${netnsName} 2> /dev/null || true
             ${ip} netns add ${netnsName}
             ${ipNetns} link set lo up
             ${ip} link add ${veth} type veth peer name ${peer}
@@ -179,7 +180,6 @@ in {
           serviceConfig = {
             Type = "oneshot";
             RemainAfterExit = "yes";
-            ExecStartPre = "-${ip} netns delete ${netnsName}";
           };
         };
       };