Move GetDuplicateSuffix to where it's used

SabreTools.DatItems/DatItem.cs

@@ -264,12 +264,6 @@ namespace SabreTools.DatItems
 
         #region Sorting and Merging
 
-        /// <summary>
-        /// Get unique duplicate suffix on name collision
-        /// </summary>
-        /// <returns>String representing the suffix</returns>
-        public virtual string GetDuplicateSuffix() => _internal.GetDuplicateSuffix();
-
         /// <summary>
         /// Get the dictionary key that should be used for a given item and bucketing type
         /// </summary>