unassign tasks should be optional / #1649

I don't think unassigning the person from all his tasks is a good idea.
For various reasons the deleted person should keep all his tasks.
just a few reasons:

• the PM needs the booked efforts with the tasks
• statistics
• If the PM has some questions on specific tasks, he should be able to call/mail the deleted person
• If the person is revoked, he/she should see his old tasks
• ...

But if the person is deleted forever and his/her old tasks are not needed anymore it should be optional to delete the person from all tasks/efforts/...