Remove unused db.api.get_ec2_instance_id_by_uuid

The last use was removed in change
I66d60a29f0d909edd381e8492a4754bdd0eb8e52:

ec2: Convert to use EC2InstanceMapping object

Change-Id: Id50c9af365c0f812862f44288a84f8da46dd007c
Merge "Remove unused db.api.get_ec2_instance_id_by_uuid"