Removed unuzed method

FourInARow/GameBoard.cs

@@ -53,13 +53,6 @@ public class GameBoard : ICloneable
         return true;
     }
 
-    public static Player Opposite(Player player) => player switch
-    {
-        Player.Human => Player.Computer,
-        Player.Computer => Player.Human,
-        _ => Player.None
-    };
-
     public Player? GetWinner()
     {
         if (!cacheInvalidated) return winnerCache;