lib/tests/test-with-nix: remove broken import ./check-eval

This doesn't do the right thing here, because it evaluates
the test with nix that is evaluating the `nixpkgs-lib-tests-nix-${nix.version}`
derivation, not the Nix/Lix under test. This was just really busted for a long
time.

lib/tests/check-eval.nix

@@ -1,10 +0,0 @@
-# Throws an error if any of our lib tests fail.
-
-let
-  tests = [
-    "misc"
-    "systems"
-  ];
-  all = builtins.concatLists (map (f: import (./. + "/${f}.nix")) tests);
-in
-if all == [ ] then null else throw (builtins.toJSON all)

lib/tests/test-with-nix.nix

@@ -18,7 +18,6 @@
 pkgs.runCommand "nixpkgs-lib-tests-nix-${nix.version}"
   {
     buildInputs = [
-      (import ./check-eval.nix)
       (import ./fetchers.nix)
       (import ../path/tests {
         inherit pkgs;