refactor: change logout() return type to void

[kenjis]

• nobody checks the return value of logout()
  • normally, the logout process cannot fail.
  • if something is wrong, it should throw an Exception.
• Events::trigger() may return false, but it does not mean errors.
  • when one listener returns false, the execution of listeners stops.
  • https://github.com/codeigniter4/CodeIgniter4/blob/ad956b426a1595a490f6b83b9e74338e0eb219e4/system/Events/Events.php#L149-L161

[MGatner]

Yes I agree. Logout should be an idempotent action, which makes any return largely unhelpful.