Looking at the code for the users endpoint, I don’t think this is possible. I also dug up this ticket on the issue which links to this ticket that confirms that this is a deliberate limitation.
The explanation is that WordPress does not give the option to delete users without either deleting their content or reassigning it to another author. This is because WordPress does not support content without an author. If this parameter were optional then it would be possible to leave behind content without an author.
It looks like there was talk about allowing deletion of a user’s content by passing a false value, but that does not appear to have been implemented.