I have a Nexus 7 2013 WiFi-only (flo) device. I rooted it and just generally messed around with it. Now I want to put it back to complete stock everything (bootloader, recovery, ROM, etc) AND COMPLETELY WIPE THE DEVICE. When following the tutorial detailed here: Tutorial, an inspection with TWRP file manager (ON A NON-ROOTED DEVICE) shows that under '/', there is still a directory called 'SuperSU' (evidence that I rooted). It's clear that wiping the device in this way is not incredibly thorough. How can I put my Nexus back to COMPLETE, wiped stock (as though I had literally just opened the box from the factory)?