trivial-builders/test/references.nix: fix eval

The command

```
nix-build -A tests.trivial-builders.references --show-trace
```

fails eval with

```
in job ‘nixpkgs.tests.trivial-builders.references’:
error: The option `meta.description' does not exist. Definition values:
       - In `makeTest parameters': "Run the Nixpkgs trivial builders tests"
```

because `meta.description` and `meta.license` are not valid for
`nixosTest`s (they are valid for `mkDerivation` of course).

This has been causing Hydra eval failures:

  https://hydra.nixos.org/jobset/nixos/pr-209870-gcc-external-bootstrap#tabs-errors

Let's fix eval by removing these attributes.

pkgs/build-support/trivial-builders/test/references.nix

@@ -45,10 +45,8 @@ testers.nixosTest {
       """)
     '';
   meta = {
-    license = lib.licenses.mit; # nixpkgs license
     maintainers = with lib.maintainers; [
       roberth
     ];
-    description = "Run the Nixpkgs trivial builders tests";
   };
 }