Remove unused using Xunit (global using in xunit v3)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tig

tests/Examples.SmokeTests/ExampleSmokeTests.cs

@@ -1,5 +1,4 @@
 using System.Diagnostics;
-using Xunit;
 
 namespace Examples.SmokeTests;