flake-modules: drop flake-root

treefmt isn't actually using flake-root to find the flake's root; it
does it itself.

All we were doing was passing flake-root's default `projectRootFile` to
treefmt, which is `"flake.nix"`;

flake.lock

@@ -71,21 +71,6 @@
         "type": "github"
       }
     },
-    "flake-root": {
-      "locked": {
-        "lastModified": 1713493429,
-        "narHash": "sha256-ztz8JQkI08tjKnsTpfLqzWoKFQF4JGu2LRz8bkdnYUk=",
-        "owner": "srid",
-        "repo": "flake-root",
-        "rev": "bc748b93b86ee76e2032eecda33440ceb2532fcd",
-        "type": "github"
-      },
-      "original": {
-        "owner": "srid",
-        "repo": "flake-root",
-        "type": "github"
-      }
-    },
     "flake-utils": {
       "inputs": {
         "systems": "systems"
@@ -211,7 +196,6 @@
         "devshell": "devshell",
         "flake-compat": "flake-compat",
         "flake-parts": "flake-parts",
-        "flake-root": "flake-root",
         "git-hooks": "git-hooks",
         "home-manager": "home-manager",
         "nix-darwin": "nix-darwin",