Remove System.Linq.Expressions dependency.

Author: NN---

src/CommandLine/Core/ReflectionExtensions.cs

@@ -4,7 +4,6 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
-using System.Linq.Expressions;
 using System.Reflection;
 using CommandLine.Infrastructure;
 using CommandLine.Text;
@@ -121,11 +120,7 @@ public static object CreateEmptyArray(this Type type)
 
         public static object GetDefaultValue(this Type type)
         {
-            var e = Expression.Lambda<Func<object>>(
-                Expression.Convert(
-                    Expression.Default(type),
-                    typeof(object)));
-            return e.Compile()();
+            return type.IsReferenceType ? null : Activator.CreateInstance(type);
         }
 
         public static bool IsMutable(this Type type)