Fix EZP-22372: Workflow cannot be edited if you remove a user and/or a group

Link: https://jira.ez.no/browse/EZP-22372

## Description
If a user or a group is deleted, when you edit a event/approve workflow, error messages are blocking edition (which is ok). But it also blocks the removal of the deleted user/group blocking the whole edition. This patch allows to delete those removed user/groups.

## Test
Manual tests