Revert "gnupg: fix build on darwin"

This reverts commit db3602b5d6a928a69cc00d837d513817bdabe1c4.

Fixed libusb build by 82d2960f15942bdf8594a400d70ca907e5b23647

pkgs/tools/security/gnupg/20.nix

···
 
   buildInputs
     = [ readline zlib libgpgerror libgcrypt libassuan libksba pth
-        openldap bzip2 curl libiconv ]
-        ++ stdenv.lib.optional (!stdenv.isDarwin) libusb;
+        openldap bzip2 libusb curl libiconv ];
 
   patchPhase = ''
     find tests -type f | xargs sed -e 's@/bin/pwd@${coreutils}&@g' -i