Revert wrong fix

Author: andrewvk

Main/src/Lazy.cs

@@ -1,6 +1,4 @@
-// Same functionality already exists in TPL for 3.5 used in FW 3.5 targeting, so we exclude it from FW3.5 version
-#if !FW35
-using System;
+using System;
 using System.Threading;
 
 using JetBrains.Annotations;
@@ -80,5 +78,4 @@ public static class Lazy
 		/// <returns>New <see cref="Lazy{T}"/> instance.</returns>
 		public static Lazy<T> Create<T>(Func<T> valueFactory, LazyThreadSafetyMode mode) => new Lazy<T>(valueFactory, mode);
 	}
-}
-#endif
+}