Fix cannot delete Dynamips router the content of the "usage" field. Fixes https://github.com/GNS3/gns3-gui/issues/2947

gns3server/compute/dynamips/__init__.py

@@ -493,7 +493,7 @@ class Dynamips(BaseManager):
             await vm.set_sparsemem(False)
 
         usage = settings.get("usage")
-        if usage and usage != vm.usage:
+        if usage is not None and usage != vm.usage:
             vm.usage = usage
 
         # update the configs if needed