Just download the factory image, unzip it, open the system.img and extract the stock boot.img
Then go into fastboot and flash the stock boot image to both slots and reboot.
This restores your stock kernel, and you're done if you just wanted to go back to stock. Now you can re-root it, and/or install another kernel.
You need to be stock before installing another kernel, so you have an untouched ramdisk.