Update src/libraries/System.Diagnostics.Process/tests/KillOnParentExitTests.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: jkotas

src/libraries/System.Diagnostics.Process/tests/KillOnParentExitTests.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Runtime.InteropServices;
-using System.Threading;
 using Microsoft.DotNet.RemoteExecutor;
 using Xunit;