nixos/startx: remove graphical-session assertions

this assertion broke gnome sessions in very hard to debug way:

- gdm starts, but on successful login just returns to login screen
- journalctl isn't exactly helpful in this condition:
  - a typical gnome login will involve many warnings and errors, that
    aren't actually preventing login, but will lead affected users
    on a merry chase for many hours
  - the actual indicators in the log arent't even an errors, only info and warning
    - graphical-session.target: Starting requested but asserts failed.
    - Assertion failed for Current graphical user session.

startx is a power tool for power users, needing a certain level of
expertise for the user to even want it, let alone use correctly.
However, the expectation is, that the necessary expertise will be
contained within the domain of startx and that it not break tools for
regular users.

This partially reverts commit e1c3082.

Author: bendlas

nixos/modules/services/x11/display-managers/startx.nix

@@ -102,14 +102,6 @@ in
 
     environment.systemPackages = with pkgs; [ xorg.xinit ];
 
-    # Make graphical-session fail if the user environment has not been imported
-    systemd.user.targets.graphical-session = {
-      unitConfig.AssertEnvironment = [
-        "DISPLAY"
-        "XDG_SESSION_ID"
-      ];
-    };
-
   };
 
 }