remove snap

1. Run 'snap list', e.g.
   Name                 Version    Rev    Tracking       Publisher   Notes
   canonical-livepatch  9.5.5      95     latest/stable  canonical✓  -
   core                 16-2.47.1  10185  latest/stable  canonical✓  core
   core18               20200929   1932   latest/stable  canonical✓  base
   kde-frameworks-5     5.47.0     27     latest/stable  kde✓        -
   pdftk                2.02-4     9      latest/stable  smoser      -
2. sudo snap remove  canonical-livepatch
3. umount /snap/core/xxxxxx, e.g. df -h shows
   /dev/loop2       98M   98M     0  100% /snap/core/10577
   /dev/loop3       98M   98M     0  100% /snap/core/10583
   Then, issue the lines:
   sudo umount /snap/core/10577
   sudo umount /snap/core/10583
    
4. snap list --all
   Name  Version    Rev    Tracking       Publisher   Notes
   core             10577  latest/stable  canonical✓  disabled,broken
   core  16-2.48.2  10583  latest/stable  canonical✓  core
   sudo snap remove core --revision 10577
   If  sudo snap remove core --revision 10583 not work, then stop snap service by
   sudo systemctl stop snapd
   then
   sudo snap remove core --revision 10583
   should work.
   
5. sudo snap remove core
6. Run 'snap list' again to see if anything left. e.g.
   No snaps are installed yet. Try 'snap install hello-world'.

7. sudo apt purge snapd
8. done.