stdenv/darwin: move secure-format patch to gnum4

This is the only derivation that needs it anymore.

pkgs/development/tools/misc/gnum4/default.nix

@@ -13,7 +13,8 @@ stdenv.mkDerivation rec {
   configureFlags = [ "--with-syscmd-shell=${stdenv.shell}" ];
 
   # Upstream is aware of it; it may be in the next release.
-  patches = [ ./s_isdir.patch ] ++ stdenv.lib.optional stdenv.hostPlatform.isDarwin stdenv.secure-format-patch;
+  patches = [ ./s_isdir.patch ]
+    ++ stdenv.lib.optional stdenv.isDarwin ./darwin-secure-format.patch;
 
   meta = {
     homepage = https://www.gnu.org/software/m4/;

pkgs/stdenv/darwin/default.nix

@@ -132,9 +132,6 @@ in rec {
         extraAttrs = {
           inherit platform;
           parent = last;
-
-          # This is used all over the place so I figured I'd just leave it here for now
-          secure-format-patch = ./darwin-secure-format.patch;
         };
         overrides  = self: super: (overrides self super) // { fetchurl = thisStdenv.fetchurlBoot; };
       };
@@ -420,9 +417,6 @@ in rec {
       inherit platform bootstrapTools;
       libc         = pkgs.darwin.Libsystem;
       shellPackage = pkgs.bash;
-
-      # This is used all over the place so I figured I'd just leave it here for now
-      secure-format-patch = ./darwin-secure-format.patch;
     };
 
     allowedRequisites = (with pkgs; [