Keep players in the PlayerTab until round end (#5921)

* Rework AdminSystem to keep players in the PlayerTab until round end

* Prevent sending PlayerStatus to deadminned admins

* Change _playerList to non-nullable

* Clean up unused usings

* Consolidate duplicate foreach loop code

Content.Shared/Administration/Events/PlayerInfoRemovalMessage.cs

@@ -1,13 +0,0 @@
-using System;
-using Robust.Shared.GameObjects;
-using Robust.Shared.Network;
-using Robust.Shared.Serialization;
-
-namespace Content.Shared.Administration.Events
-{
-    [NetSerializable, Serializable]
-    public class PlayerInfoRemovalMessage : EntityEventArgs
-    {
-        public NetUserId NetUserId;
-    }
-}