Fix 'config run' by removing default mount.

In, pull #1822 mount logic was refactored where the default
mount using zero-value no longer makes sense and leads to
this failure:

"invalid argument "type=,src=,dst=:ro" for "--mount" flag: type is
required"

I think the intent here was to remove default mount.

kyaml/runfn/runfn_test.go

@@ -40,10 +40,7 @@ kind:
 		return
 	}
 	filter := instance.containerFilterProvider("example.com:version", "", api)
-	defaultMount := filters.StorageMount{}
-	mounts := []filters.StorageMount{}
-	mounts = append(mounts, defaultMount)
-	assert.Equal(t, &filters.ContainerFilter{Image: "example.com:version", Config: api, StorageMounts: mounts}, filter)
+	assert.Equal(t, &filters.ContainerFilter{Image: "example.com:version", Config: api}, filter)
 }
 
 func TestCmd_Execute(t *testing.T) {