UI: Tree view, no longer auto-expand clicked item

When clicking on an expandable item in the tree view, it used to
automatically expand that item. This has now been removed; clicking just
makes that item the active one.

This impacts all tree views. The authors of the following ones have been
consulted on this, and all agreed it was a good idea:

- Asset Catalogs
- Bone Collections
- Grease Pencil Layers
- Node Panels
- Light Linking (uses tree view, but just to present a flat list)

source/blender/editors/interface/views/tree_view.cc

@@ -302,9 +302,6 @@ void AbstractTreeViewItem::tree_row_click_fn(bContext *C, void *but_arg1, void *
   AbstractTreeViewItem &tree_item = reinterpret_cast<AbstractTreeViewItem &>(*item_but->view_item);
 
   tree_item.activate(*C);
-  /* Not only activate the item, also show its children. Maybe this should be optional, or
-   * controlled by the specific tree-view. */
-  tree_item.set_collapsed(false);
 }
 
 void AbstractTreeViewItem::add_treerow_button(uiBlock &block)