lib/systems/doubles: drop x86_64-darwin

This ensures that `lib.platforms.darwin` doesn’t include
`x86_64-darwin` and that the unsupported platform check will trigger
even once the bespoke deprecation error is removed. It doesn’t break
`lib.systems.elaborate "x86_64-darwin"`.

doc/stdenv/cross-compilation.chapter.md

@@ -50,7 +50,7 @@ The exact schema these fields follow is a bit ill-defined due to a long and conv
 
 `system`
 
-: This is a two-component shorthand for the platform. Examples of this would be "x86_64-darwin" and "i686-linux"; see `lib.systems.doubles` for more. The first component corresponds to the CPU architecture of the platform and the second to the operating system of the platform (`[cpu]-[os]`). This format has built-in support in Nix, such as the `builtins.currentSystem` impure string.
+: This is a two-component shorthand for the platform. Examples of this would be "aarch64-darwin" and "i686-linux"; see `lib.systems.doubles` for more. The first component corresponds to the CPU architecture of the platform and the second to the operating system of the platform (`[cpu]-[os]`). This format has built-in support in Nix, such as the `builtins.currentSystem` impure string.
 
 `config`
 

lib/systems/doubles.nix

@@ -19,7 +19,6 @@ let
     "x86_64-cygwin"
 
     # Darwin
-    "x86_64-darwin"
     "aarch64-darwin"
 
     # FreeBSD

lib/tests/systems.nix

@@ -127,7 +127,6 @@ lib.runTests (
     ];
     testx86_64 = mseteq x86_64 [
       "x86_64-linux"
-      "x86_64-darwin"
       "x86_64-freebsd"
       "x86_64-genode"
       "x86_64-redox"
@@ -145,7 +144,6 @@ lib.runTests (
       "x86_64-cygwin"
     ];
     testdarwin = mseteq darwin [
-      "x86_64-darwin"
       "aarch64-darwin"
     ];
     testfreebsd = mseteq freebsd [