nexusmods-app: drop dead avalonia code

We briefly depended on the avalonia nixpkgs package instead of fetching
it from nuget, however since disabling it we have never re-enabled it.

(cherry picked from commit 9c85f55e72)

pkgs/by-name/ne/nexusmods-app/package.nix

@@ -1,6 +1,5 @@
 {
   _7zz,
-  avalonia,
   buildDotnetModule,
   callPackage,
   desktop-file-utils,
@@ -45,11 +44,6 @@ buildDotnetModule (finalAttrs: {
   projectFile = "src/NexusMods.App/NexusMods.App.csproj";
   testProjectFile = "NexusMods.App.sln";
 
-  buildInputs = [
-    # TODO: bump avalonia to 11.1.3
-    # avalonia
-  ];
-
   nativeCheckInputs = [ _7zz ];
 
   nativeBuildInputs = [