Replace IResettingEntitySystem with RoundRestartCleanupEvent. (#4245)

* Replace IResettingEntitySystem with RoundRestartCleanupEvent.

* oops

Content.Shared/GameTicking/IResettingEntitySystem.cs

@@ -1,9 +0,0 @@
-#nullable enable
-
-namespace Content.Shared.GameTicking
-{
-    public interface IResettingEntitySystem
-    {
-        void Reset();
-    }
-}

Content.Shared/GameTicking/RoundRestartCleanupEvent.cs

@@ -0,0 +1,13 @@
+using System;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Serialization;
+
+#nullable enable
+
+namespace Content.Shared.GameTicking
+{
+    [Serializable, NetSerializable]
+    public class RoundRestartCleanupEvent : EntityEventArgs
+    {
+    }
+}