nits: doc update, test cleanup (#3520)

src/OpenTelemetry.Instrumentation.Http/README.md

@@ -46,6 +46,7 @@ requires adding the package
 to the application.
 
 ```csharp
+using OpenTelemetry;
 using OpenTelemetry.Trace;
 
 public class Program

test/OpenTelemetry.Instrumentation.Http.Tests/HttpClientTests.netcore31.cs

@@ -18,7 +18,6 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
-using System.IO;
 using System.Linq;
 using System.Net.Http;
 using System.Reflection;