[edit controller] fix handling of removal of subentities from an edit form In CubicWeb 3.18 the semantics of dropping a composite relation were changed. It had before the effect of deleting the subentities. Now the edit controller must handle this by itself (it was not adapted then by mistake). So if it detects the removal of a composite relation (on the browser side, the user clicked on the "remove" action), the subentity is now explicitly deleted. It was chosen not to change the form generation but the EditController: - to ensure pre-3.18 forms backward compat - to avoid the introduction of a specific entity-deletion API (RQL is probably our best API for complex DB operations) Related to #8529868.