meta: Replace treefmt with flake formatter output

This makes the code base more portable by *only* depending on Nix as a
dependency.

Note that the code base hasn't been migrated to nixfmt-rfc-style yet.

flake.nix

@@ -46,6 +46,8 @@
         ];
       };
     };
+
+    formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixpkgs-fmt;
   } //
     (builtins.listToAttrs
       (builtins.map

treefmt.toml

@@ -1,9 +0,0 @@
-[formatter.deno]
-command = "deno"
-options = ["fmt"]
-includes = [ "*.ts" ]
-
-[formatter.nix]
-command = "nixpkgs-fmt"
-includes = [ "*.nix" ]
-excludes = [ "./hardware/*" ]