displaylink: fix tests eval

Before the change `tests` eval failed for non-existent `displaylink`
nixos test as:

    $ nix build --no-link -f. displaylink.tests
    error: attribute 'displaylink' missing
               72|   passthru = {
               73|     tests = {
                 |              ^
               74|       inherit (nixosTests) displaylink;

As far as I can see `displaylink` was never added to `nixos/tests/all-tests.nix`.

pkgs/os-specific/linux/displaylink/default.nix

@@ -8,7 +8,6 @@
 , makeWrapper
 , requireFile
 , substituteAll
-, nixosTests
 }:
 
 let
@@ -69,12 +68,6 @@ stdenv.mkDerivation rec {
   dontStrip = true;
   dontPatchELF = true;
 
-  passthru = {
-    tests = {
-      inherit (nixosTests) displaylink;
-    };
-  };
-
   meta = with lib; {
     description = "DisplayLink DL-5xxx, DL-41xx and DL-3x00 Driver for Linux";
     homepage = "https://www.displaylink.com/";