all-packages: never inherit libiconv from darwin

libiconv and darwin.libiconv are the same thing on Darwin, but using
darwin.libiconv means that packages will necessarily not be correct
for other platforms where we don't include libiconv in libc, like
FreeBSD.

pkgs/top-level/all-packages.nix

···
     description = mame.meta.description + " (tools only)";
   } (lib.getOutput "tools" mame);
 
-  mednafen = callPackage ../applications/emulators/mednafen {
-    inherit (darwin) libiconv;
-  };
+  mednafen = callPackage ../applications/emulators/mednafen { };
 
   mednafen-server = callPackage ../applications/emulators/mednafen/server.nix { };
 
···
   procs = darwin.apple_sdk_11_0.callPackage ../tools/admin/procs {
     inherit (darwin.apple_sdk_11_0.frameworks) Security;
     inherit (darwin.apple_sdk_11_0) Libsystem;
-    inherit (darwin) libiconv;
   };
 
   procyon = callPackage ../tools/misc/procyon { };
···
   };
   cargo-fuzz = callPackage ../development/tools/rust/cargo-fuzz { };
   cargo-geiger = callPackage ../development/tools/rust/cargo-geiger {
-    inherit (darwin) libiconv;
     inherit (darwin.apple_sdk.frameworks) Security CoreFoundation;
   };
 
···
   rbenv = callPackage ../development/ruby-modules/rbenv { };
 
   inherit (callPackage ../development/interpreters/ruby {
-    inherit (darwin) libiconv libobjc libunwind;
+    inherit (darwin) libobjc libunwind;
     inherit (darwin.apple_sdk.frameworks) Foundation;
   })
     mkRubyVersion
···
 
   libdeltachat = callPackage ../development/libraries/libdeltachat {
     inherit (darwin.apple_sdk.frameworks) CoreFoundation Security;
-    inherit (darwin) libiconv;
   };
 
   libdevil = callPackage ../development/libraries/libdevil {
···
 
   libxml2 = callPackage ../development/libraries/libxml2 {
     python = python3;
-    inherit (darwin) libiconv;
   };
 
   libxml2Python = let
···
     wlgreet = callPackage ../applications/display-managers/greetd/wlgreet.nix { };
   };
 
-  goldendict = libsForQt5.callPackage ../applications/misc/goldendict {
-    inherit (darwin) libiconv;
-  };
+  goldendict = libsForQt5.callPackage ../applications/misc/goldendict { };
 
   gomuks = callPackage ../applications/networking/instant-messengers/gomuks { };
 
···
 
   jujutsu = callPackage ../applications/version-management/jujutsu {
     inherit (darwin.apple_sdk.frameworks) Security SystemConfiguration;
-    inherit (darwin) libiconv;
   };
 
   nbstripout = callPackage ../applications/version-management/nbstripout { };
···
   moon-buggy = callPackage ../games/moon-buggy { };
 
   inherit (callPackages ../games/minetest {
-    inherit (darwin) libiconv;
     inherit (darwin.apple_sdk.frameworks) OpenGL OpenAL Carbon Cocoa;
   })
     minetestclient_5 minetestserver_5;