I learned that Elastic IP Address allocations are a scarce resource on AWS - the default limit is only 5 per region(!). According to the documentation, here's the reasoning behind this small limit:

By default, all AWS accounts are limited to 5 EIPs, because public (IPv4) Internet addresses are a scarce public resource. We strongly encourage you to use an Elastic IP address primarily for the ability to remap the address to another instance in the case of instance failure, and to use DNS hostnames for all other inter-node communication.

We can submit a limit increase if needed, but for now since failover isn't necessary for standalone adhoc instances, this PR removes the Elastic IP resource for this server type.