Revert "Add packageTests"

This reverts commit 87b3740d66.

packageTests wasn't inherently wrong, but I am not fully convinced
that it is necessary. Specifically, it would only be used by:

 - `nix-build`, which is going to be a legacy command in the mid term
 - ofborg, perhaps, not even implemented

whereas the following use cases do not need it:

 - `nix build` (experimental command with a future, unlike `nix-build`)
 - `release.nix` (Hydra), might use similar logic but not reuse this

Further, considering that this is only necessary for tests of
functions, I am hesitant to introduce it.
Meanwhile, it should not hinder the rest of the PR.

pkgs/top-level/all-packages.nix

@@ -119,24 +119,6 @@ with pkgs;
     callTest = t: t.test;
   };
 
-  ### Traversal for package tests
-
-  # Whereas most tests can be run with `nix-build -A my-package.tests`, this
-  # does not work for tests on a __functor attrset.
-  # TODO: enable these on hydra.nixos.org?
-  packageTests = let
-    getTests = attrs:
-      if builtins.typeOf attrs != "set"
-      then {}
-      else if lib.isDerivation attrs
-        then lib.recurseIntoAttrs attrs.tests or {}
-        else if attrs?__functor
-        then lib.recurseIntoAttrs attrs.tests or {}
-        else lib.mapAttrs (k: if k == "recurseForDerivations" then v: v else getTests) attrs;
-    in
-      getTests pkgs;
-
-
   ### BUILD SUPPORT
 
   auditBlasHook = makeSetupHook