Cleaned up V13 related and obsoleted system directory constants (#16133)

* Cleaned up V13 related and obsoleted system directory constants

* The LogFiles constant is still used, it seems

Author: kjac

src/Umbraco.Core/Constants-SystemDirectories.cs

@@ -37,16 +37,12 @@ public static class SystemDirectories
 
         public const string TempFileUploads = TempData + "/FileUploads";
 
-        public const string TempImageUploads = TempFileUploads + "/rte";
-
         public const string Install = "~/install";
 
         public const string AppPlugins = "/App_Plugins";
 
         public const string BackOfficePath = "/umbraco/backoffice";
 
-        public const string PluginIcons = "/backoffice/icons";
-
         public const string MvcViews = "~/Views";
 
         public const string PartialViews = MvcViews + "/Partials/";
@@ -62,8 +58,5 @@ public static class SystemDirectories
         /// </summary>
         [Obsolete("Use LoggingSettings.GetLoggingDirectory() instead, will be removed in Umbraco 13.")]
         public const string LogFiles = Umbraco + "/Logs";
-
-        [Obsolete("Use PluginIcons instead")]
-        public static string AppPluginIcons => "/Backoffice/Icons";
     }
 }