Remove ability for sysops to add themself to flood on enwikibooks
This is unnecassary, because they can add flood to any account.
This is a follow-up to I3a53c6ec71964cf9ea88a321cf2ce26942d4cd3c.
Bug: T285594
Change-Id: If343273ad976eaf7f6009e850b4100bba8d0a9b6