Well, it is safe ATM to let it update, actually. The current released jailbreak is pretty resilient.
For another thing, you are on the latest currently-existing firmware.
You can enable WiFi, then select the currently-connected network and choose to "Forget" it.
What files to delete if you must do it via the shell?
Well, pretty much anything in the system partition that has user-specific data, is in /var/local/
I *think* it (WiFi logins) is contained in /var/local/system/wifid.conf