Fixed EZP-20636 - Restoring a removed object to the original location is always unavailable

This issue is a regression from 77c2c4d8f950730c47c75adb1b217c63f312e678 (fix for [EZP-20492](https://jira.ez.no/browse/EZP-20636).

This patch avoids using `eznode_assignment` for restoring content objects and uses `ezcontentobject_trash` instead.