pypy2Packages.attrs: fix tests eval

Without the change `tests` eval fails as:

    $ nix build --no-link -f. pypy2Packages.attrs.tests
    error:
       error: path 'pkgs/development/python2-modules/attrs/tests.nix' does not exist

pkgs/development/python2-modules/attrs/default.nix

@@ -32,10 +32,6 @@ buildPythonPackage rec {
   # Instead, we do this as a passthru.tests test.
   doCheck = false;
 
-  passthru.tests = {
-    pytest = callPackage ./tests.nix { };
-  };
-
   meta = with lib; {
     description = "Python attributes without boilerplate";
     homepage = "https://github.com/hynek/attrs";