code/extensions/FusionExtension.php
public function onAfterDelete() {

		parent::onAfterDelete();
		$fusion = FusionTag::get()->byID($this->owner->FusionTagID);
		$types = unserialize($fusion->TagTypes);
		unset($types[$this->owner->ClassName]);
		$fusion->TagTypes = !empty($types) ? serialize($types) : null;
		$fusion->write();
	}
public function onAfterDelete() {

		parent::onAfterDelete();
		$fusion = FusionTag::get()->byID($this->owner->FusionTagID);
		$types = unserialize($fusion->TagTypes);
		unset($types[$this->owner->ClassName]);
		$fusion->TagTypes = !empty($types) ? serialize($types) : null;
		$fusion->write();
	}
Update the fusion tag to remove this tag type.