libiconv: make default on Darwin

The Darwin libiconv tries to be compatible with GNU libiconv, but it’s
not. Recent versions of Autoconf and gnulib include checks for issues in
Darwin’s libiconv implementation, which has effectively turned
`autoreconfHook` into `autoBreakDarwinHook` due to failing to link
libiconv. Instead of continuing to work around it, make GNU libiconv the
default. With the UTF-8-MAC patch, it should be a drop-in replacement.

Author: reckenrode

pkgs/top-level/all-packages.nix

@@ -6676,8 +6676,6 @@ with pkgs;
       ]
     then
       libcIconv pkgs.libc
-    else if stdenv.hostPlatform.isDarwin then
-      darwin.libiconv
     else
       libiconvReal;