gpg/pth: fixing the pth building on arm

I could built pth with the fix commented in:
http://forums.gentoo.org/viewtopic-t-904280-start-0.html

And I remove the 'pth.supported' structure I had built.

pkgs/development/libraries/libassuan/default.nix

@@ -8,8 +8,7 @@ stdenv.mkDerivation rec {
     sha256 = "06xckkvxxlx7cj77803m8x58gxksap4k8yhspc5cqsy7fhinimds";
   };
 
-  propagatedBuildInputs = [ libgpgerror ]
-    ++ stdenv.lib.optional pth.supported pth;
+  propagatedBuildInputs = [ libgpgerror pth ];
 
   doCheck = true;