Move manager Update methods to be explicit to where they are still used by legacy code

Also fixes skin hash repopulation being completely broken.

osu.Game/Database/IModelManager.cs

@@ -12,13 +12,6 @@ namespace osu.Game.Database
     public interface IModelManager<TModel>
         where TModel : class
     {
-        /// <summary>
-        /// Perform an update of the specified item.
-        /// TODO: Support file additions/removals.
-        /// </summary>
-        /// <param name="item">The item to update.</param>
-        void Update(TModel item);
-
         /// <summary>
         /// Delete an item from the manager.
         /// Is a no-op for already deleted items.