When reinstalling LXD today after having some problems, I ran lxd init and accepted all of the defaults, and then lxd init exited with this message:

Error: Failed to create storage pool 'default': Failed to create the ZFS pool: cannot create 'default': pool already exists

Okay, so the previous storage pool was already there. 

I'll confirm this by running:

zpool list

... which returned this output... 

NAME      SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
default  28.8G   342M  28.4G         -     1%     1%  1.00x  ONLINE  -

I see that the pool named "default" is indeed there.

So to get rid of it, all that is required is to run....

zpool destroy default

Whether it was removed or not can be confirmed by again running:

zpool list