[FIX] Prevent update from trying to remove already removed records

When modifying an xml data file of a module, if update implies removing datas that are linked by a many2one with an ondelete=cascade, the update tries to remove every record that is no more in the xml even if it already has been removed.

Example:

xml data file contains a product.product. The update automatically creates a product.template too.
when removing the product from the wml file and updating the module, the process_end method on ir_model removes the template (which removes the product too) and then tries to remove the product which has already been erased from the db