Ubuntu 16.04 Core OS snap won't remove
Solution 1:
You can't remove the ubuntu-core snap using the snap command, no. If you really want to remove it, remove the snapd package itself and then remove the /var/lib/snapd directory (if it says it's still in use, unmount the snap and try again).
Solution 2:
In my case,I found this page at ubuntuforums.
So I simply executed this line-command:
sudo apt purge snapd ubuntu-core-launcher squashfs-tools