To clarify, though your question has been answered. However not as straight to the point as I feel you would've liked.... Deleting the Rom from your SD card will in NO way do anything to revert your current operating system back to its factory stock setting. It simply does one thing deletes a file from your SD card. No, you can not simply delete a ROM that has been flashed, because it replaces your entire system, just like running a new OS. Deleting that (formatting) in a recovery would simply leave you with a blank phone, with no system to boot up. You need a system to boot up and with this phone you have to use the unbrick method everytime to revert (flash) back to stock and that's because of the locked boot loader and the recovery being on a different partition due to the locked boot loader. On other phones without locked bootloaders, one could simply go back into recovery and flash a zip to go back entirely to stock and remove root. That's what you get with a locked boot loader, so I hope that clears things up a bit