GP: Remove Layer order userprefs parameter

After a lot of discussion about this option (see 18f1175940) we have decided set always the order of GP layers in 2D mode (Top->Down) and remove the parameter from User Preferences screen.

Internally all works equal, but in the UI the list is inverted.

The filter buttons to reverse the list or sort alphabetically have been removed because these buttons are not logic in this context.

release/scripts/startup/bl_ui/space_topbar.py

@@ -352,15 +352,12 @@ class TOPBAR_PT_gpencil_layers(Panel):
             self.draw_layers(context, layout, gpd)
 
     def draw_layers(self, context, layout, gpd):
-        userpref = context.user_preferences
-        edit = userpref.edit
-        reverse = edit.use_grease_pencil_reverse_layers
         row = layout.row()
 
         col = row.column()
         layer_rows = 10
         col.template_list("GPENCIL_UL_layer", "", gpd, "layers", gpd.layers, "active_index",
-                          rows=layer_rows, reverse=reverse)
+                          rows=layer_rows, reverse=True)
 
         col = row.column()