.devcontainer: apply nixfmt-rfc-style on save

formatterPath does not support the specifier "nixfmt-rfc-style"

92fdb077e9/package.json (L76-L85)

Currently it is just ignored,
because of we are setting LSP.

92fdb077e9/README.md (L63)

This commit removes the unused config
and enables auto formatter on save.

.devcontainer/devcontainer.json

@@ -15,7 +15,9 @@
         "jnoortheen.nix-ide"
       ],
       "settings": {
-        "nix.formatterPath": "nixfmt-rfc-style",
+        "[nix]": {
+            "editor.formatOnSave": true
+        },
         "nix.enableLanguageServer": true,
         "nix.serverPath": "nixd"
       }