nixVersion.nix_{2_6,2_7,2_8,2_9}: remove

the comment from 2_6 does not apply anymore because the fix has been
merged nix#7283
and the fix has been applied to 2_9+

pkgs/tools/package-management/nix/default.nix

@@ -59,47 +59,13 @@ in lib.makeExtensible (self: {
 
   nix_2_5 = throw "nixVersions.nix_2_5 has been removed";
 
-  # This is the last version of nix where https://github.com/NixOS/nix/issues/6572
-  # is not an unsolved breaking bug. Please keep it until the issue is fixed
-  # and in latest stable nix.
-  nix_2_6 = common {
-    version = "2.6.1";
-    sha256 = "sha256-E9iQ7f+9Z6xFcUvvfksTEfn8LsDfzmwrcRBC//5B3V0=";
-  };
+  nix_2_6 = throw "nixVersions.nix_2_6 has been removed";
 
-  nix_2_7 = common {
-    version = "2.7.0";
-    sha256 = "sha256-m8tqCS6uHveDon5GSro5yZor9H+sHeh+v/veF1IGw24=";
-    patches = [
-      # remove when there's a 2.7.1 release
-      # https://github.com/NixOS/nix/pull/6297
-      # https://github.com/NixOS/nix/issues/6243
-      # https://github.com/NixOS/nixpkgs/issues/163374
-      (fetchpatch {
-        url = "https://github.com/NixOS/nix/commit/c9afca59e87afe7d716101e6a75565b4f4b631f7.patch";
-        sha256 = "sha256-xz7QnWVCI12lX1+K/Zr9UpB93b10t1HS9y/5n5FYf8Q=";
-      })
-    ];
-  };
+  nix_2_7 = throw "nixVersions.nix_2_7 has been removed";
 
-  nix_2_8 = common {
-    version = "2.8.1";
-    sha256 = "sha256-zldZ4SiwkISFXxrbY/UdwooIZ3Z/I6qKxtpc3zD0T/o=";
-  };
+  nix_2_8 = throw "nixVersions.nix_2_8 has been removed";
 
-  nix_2_9 = common {
-    version = "2.9.2";
-    sha256 = "sha256-uZCaBo9rdWRO/AlQMvVVjpAwzYijB2H5KKQqde6eHkg=";
-    patches = [
-      (fetchpatch {
-        # https://github.com/NixOS/nix/pull/7283
-        name = "fix-requires-non-existing-output.patch";
-        url = "https://github.com/NixOS/nix/commit/3ade5f5d6026b825a80bdcc221058c4f14e10a27.patch";
-        sha256 = "sha256-s1ybRFCjQaSGj7LKu0Z5g7UiHqdJGeD+iPoQL0vaiS0=";
-      })
-      patch-sqlite-exception
-    ];
-  };
+  nix_2_9 = throw "nixVersions.nix_2_9 has been removed";
 
   nix_2_10 = common {
     version = "2.10.3";