Fix crash when selecting re-added TreeItem::Cell

[mihe]

Fixes #88889.

When removing a TreeItem from a Tree it doesn't clear the selected property of the cells in the TreeItem, which means that if you remove this TreeItem from the Tree and then add it back again, and then select the same item/cell, you wouldn't pass this condition:

godot/scene/gui/tree.cpp

Lines 2664 to 2668 in a586e86

	if (!selected_cell.selected || allow_reselect) {
	selected_cell.selected = true;
	selected_item = p_selected;
	selected_col = i;

Which then left selected_item at nullptr and would subsequently crash here:

godot/scene/gui/tree.cpp

Line 3736 in a586e86

Rect2 rect = get_selected()->get_meta("__focus_rect");

This PR fixes this by simply clearing the selected property of all the item's cells when it's removed from the tree.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.2.2.