Fix content.integration tests warnings (#17817)

Co-authored-by: metalgearsloth <31366439+metalgearsloth@users.noreply.github.com>

Content.IntegrationTests/Tests/GameRules/StartEndGameRulesTest.cs

@@ -1,9 +1,6 @@
-using System;
 using System.Linq;
-using System.Threading.Tasks;
 using Content.Server.GameTicking;
 using Content.Shared.CCVar;
-using NUnit.Framework;
 using Robust.Shared.Configuration;
 using Robust.Shared.GameObjects;