Cleanup: Remove unnecessary tree view function override, add comment

`AbstractTreeViewItem::supports_collapsing()` returns true in the
default implementation, so no need to override that to do the same.
Notes the default behavior in a comment now.

source/blender/editors/include/UI_tree_view.hh

@@ -228,6 +228,7 @@ class AbstractTreeViewItem : public AbstractViewItem, public TreeViewItemContain
 
   /**
    * Return whether the item can be collapsed. Used to disable collapsing for items with children.
+   * The default implementation returns true.
    */
   virtual bool supports_collapsing() const;
 

source/blender/editors/interface/interface_template_grease_pencil_layer_tree.cc

@@ -251,11 +251,6 @@ class LayerGroupViewItem : public AbstractTreeViewItem {
     build_layer_group_buttons(*sub);
   }
 
-  bool supports_collapsing() const override
-  {
-    return true;
-  }
-
   bool supports_renaming() const override
   {
     return true;