fix(tools/depotfmt.nix): double specification of edition

There already is a rustfmt.toml in the depot root specifying the edition.
Removing the option enables projects in the depot to update to newer
editions independently.

Fixes: https://b.tvl.fyi/issues/442
Change-Id: I15d30f69040a4ec327fd099c508e5c11cec2aecc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13347
Reviewed-by: thk <thomas@koch.ro>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-on: https://cl.snix.dev/c/snix/+/30342
Autosubmit: Florian Klink <flokli@flokli.de>
Tested-by: besadii
Reviewed-by: Domen Kožar <domen@cachix.org>

tools/depotfmt.nix

@@ -18,7 +18,6 @@ let
 
     [formatter.rust]
     command = "${pkgs.rustfmt}/bin/rustfmt"
-    options = ["--edition", "2021"]
     includes = [ "*.rs" ]
 
     [formatter.editorconfig]