Fix C# warning in Comparer_get_Default (#48643)

src/tests/JIT/opt/Devirtualization/Comparer_get_Default.cs

@@ -237,7 +237,7 @@ public struct Struct1 : IComparable
 {
     public long a;
     public long b;
-    public int CompareTo(object? obj)
+    public int CompareTo(object obj)
     {
         return b.CompareTo(((Struct1) obj).b);
     }