[Hunter] Explosive Shot update

Explosive Shot no longer explodes on ticks, but explodes once on expiry, with refreshes triggering an immediate explosion and leaving one tick for that expiry. BM casts with Explosive Venom carry their state from cast into the explosion, presumably the same would be the case for SV explosions from sources that have variable effectiveness.
Should clean up the get_background_action uses related to Explosive Shot mentioned in #9142.