Revert "libassuan: Use automatically detected libgpg-error"

This reverts commit 2d44dc9.

Cross-compilation only works if we set --with-libgpg-error-prefix,
which is why that flag was there in the first place[1].  Unfortunately
this was not noted in the commit message that added it, which made it
difficult to discover…

[1]: NixOS#57815

pkgs/development/libraries/libassuan/default.nix

@@ -13,7 +13,11 @@ stdenv.mkDerivation rec {
   outputBin = "dev"; # libassuan-config
 
   depsBuildBuild = [ buildPackages.stdenv.cc ];
-  buildInputs = [ npth gettext libgpg-error ];
+  buildInputs = [ npth gettext ];
+
+  configureFlags = [
+    "--with-libgpg-error-prefix=${libgpg-error.dev}"
+  ];
 
   doCheck = true;