I decided to put a little more effort into this so it's not deleting everything every time we update the adlist. for me this is part of my update script to keep my pihole fresh. this will only remove or add sites from adlist if needed...
#!/bin/bash
# exit on any error
set -e
# udpate raspbian
sudo apt-get update -q
sudo apt-get dist-upgrade -qy
sudo apt-get autoremove -qy
sudo apt-get autoclean -q
sudo ldconfig
# update pihole
pihole -up
# update pihole blocklists
sqlite3 /etc/pihole/gravity.db "SELECT Address FROM adlist" |sort >/home/pi/pihole.list
wget -qO - https://v.firebog.net/hosts/lists.php?type=tick |sort >/home/pi/firebog.list
comm -23 pihole.list firebog.list |xargs -I{} sudo sqlite3 /etc/pihole/gravity.db "DELETE FROM adlist WHERE Address='{}';"
comm -13 pihole.list firebog.list |xargs -I{} sudo sqlite3 /etc/pihole/gravity.db "INSERT INTO adlist (Address,Comment,Enabled) VALUES ('{}','firebog, added `date +%F`',1);"
pihole restartdns reload-lists
pihole -g