If you manually removed the interface and the LXD config for it still exists then restarting LXD service will re-create the bridge interface.
You cannot run another DNS/DHCP service on the same host as LXD (such as pi-hole) if it is listening on the wildcard port 53 (DNS) or 67 (DHCP).
You should either move pi-hole into a container, or look at tweaking the pi-hole config so it only listens on the network interfaces that LXD doesn’t managed (i.e not lxdbr0).