Finish _delete_instance() object conversion

The _delete_instance() helper has been using objects for a while
now, but was still using an old conductor method to actually
destroy the instance. This makes it use instance.destroy(). Since the
same thing happens under the covers, none of the tests need to be
changed.

Related to blueprint compute-manager-objects-juno

Change-Id: I413394bbc931d1011c276454ecb34ddd8d948f07