opam: use aspcud on darwin

aspcud is the recommended solver for OPAM and the `aspcud` package is
now building properly on Darwin. As such, we can remove the special case
for Darwin that required OPAM to fall back to the built-in solver.

pkgs/development/tools/ocaml/opam/default.nix

···
 { stdenv, lib, fetchgit, fetchurl, makeWrapper,
-  ocaml, unzip, ncurses, curl,
-  aspcudSupport ? !stdenv.isDarwin, aspcud
+  ocaml, unzip, ncurses, curl, aspcud
 }:
 
 assert lib.versionAtLeast ocaml.version "3.12.1";
···
   # Dirty, but apparently ocp-build requires a TERM
   makeFlags = ["TERM=screen"];
 
-  postInstall =
-    if aspcudSupport then ''
-      wrapProgram $out/bin/opam \
-        --suffix PATH : ${aspcud}/bin
-    ''
-    else "";
+  postInstall = ''
+    wrapProgram $out/bin/opam \
+      --suffix PATH : ${aspcud}/bin
+  '';
 
   doCheck = false;