Unbreakable elements were being considered as unremovable in some deleteBackward cases. With these changes it is now possible to remove a unbreakble element if it is empty. task-2580158 (cherry picked from commit 18abf537f07bd9c504dc19c3fa8eda5b1f4d4e9e) closes odoo/odoo#73592 Signed-off-by: Antoine Guenet <Zinston@users.noreply.github.com>