I do agree with kkboy that you are not 'required' to request a SIL from the SSS and since USCIS has your immigration records, they should be aware of your status. But remember that its the USCIS that you are dealing with. There have been several posts on this forum where in addition to some applicants who were not required to register with the SSS (just like you), even some female candidates were asked to bring SIL for their interview (go figure).
You can send the N-400 without the SIL but just to cover your bases, get the SIL for the interview (just in case and keep it as a backup document). Its so easy to get the SIL as they simply ask for your immigration records/I-94/arrival stamps.