Remove attendees from an existing training event.

Remove one or more attendees from the training's attendees list. If workbenches have already been created, the workbench simulations belonging to the removed attendees will be shut down and destroyed. Not allowed after the training's `sim_end_time` has passed. **Required roles (all):** `air_instructor`, `air_org_admin`, `user_admin` **Additional permissions:** - Only the instructor who created this training may perform this action. - The client must have the NCA 'Administrator' role set in the NCA account associated with the NGC Organization.