Remove unnecessary cleanup in test

The test test_rollback_live_migration_at_destination_correctly
makes use of a fake instance object. No need to clean this from
the database with db.instance_destroy.

This issue was also raised in:
https://review.openstack.org/#/c/90801/3/nova/tests/compute/test_compute.py

Change-Id: I66a9f60b9e7b2f69f79976c03e4e9ddcc5e02993
Merge "Remove unnecessary cleanup in test"