InnoDB fails to clean up useless B-tree pages

The test case of MDEV-14126 demonstrates that InnoDB can end up with an index tree where a non-leaf page has only one child page. Such internal nodes are totally unused and should be removed when the B-tree is being shrunk in the first place. The functions btr_discard_only_page_on_level(), btr_lift_page_up(), or btr_compress() or their callers may have to be revised.