4094cd207a
No user visible changes expected. Part of #96713, continuation of work started in249e4df110and2e221de4ce. Refer to these for a motivation and design overview. Removes `isExpandValid()` from `AbstractTreeElement` and its sub-classes as all ID elements are ported to new tree-element code design. It was meant to be temporary for this transition phase. Pull Request: https://projects.blender.org/blender/blender/pulls/109380