If the lxd/lxc knows about the bridge (i.e. it's managed) then run the following:
lxc network delete lxdbr0
If it's no longer managed by lxd/lxc, you need to run the following commands:
ip link set lxdbr0 down
ip link delete lxdbr0 type bridge
You may need to kill the LXD process listening on port 8443
(it requires sudo as lxd runs as root)
sudo lsof -i :8443