locations: Filter out network volume locations

This is another spot where I added logic to mount handling and forgot
to add it for volumes too. We don't want to show network locations,
so we need to filter out both volumes and mounts.

Fixed #1031

locations.js

@@ -162,6 +162,10 @@ var Removables = class DashToDock_Removables {
             return;
         }
 
+        if (volume.get_identifier('class') == 'network') {
+            return;
+        }
+
         let activationRoot = volume.get_activation_root();
         if (!activationRoot) {
             // Can't offer to mount a device if we don't know