Revert "feat: Add #nullable disable to generated C# code to avoid war…

…nings (#2444)" (#2786)

This reverts commit ecd825e.

The `#nullable enable` annotation is a recent C# feature, so that commit
broke compatibility with some client code.

Fixes #2781

Source/DafnyCore/Compilers/Compiler-Csharp.cs

@@ -86,7 +86,6 @@ protected override void EmitHeader(Program program, ConcreteSyntaxTree wr) {
       wr.WriteLine("// Optionally, you may want to include compiler switches like");
       wr.WriteLine("//     /debug /nowarn:162,164,168,183,219,436,1717,1718");
       wr.WriteLine();
-      wr.WriteLine("#nullable disable");
       wr.WriteLine("using System;");
       wr.WriteLine("using System.Numerics;");
       wr.WriteLine("using System.Collections;");