nixos/desktopManager: Only trace if the default session isn't found

The default session might be found in `extraSessionFilePackages`, but it's not
viable to detect at evaluation time, so emit a warning.

In LightDM instead of checking `defaultSessionName` against
`displayManager.session.names` we rely on the assertions in
`desktopManager` and `windowMananger` and just check that there's at least one
default set. The second assertion could never actually be triggered.