CE-2437: Clean up after NetBox integration

The endpoint responsible for the CSentry -> NetBox host ID migration has been removed. The host_external_id column representing the CSentry host ID, as well as the netbox_migration_log column have been removed from the database. I have retained the last_known_host_id field because in some cases it differs from the host_name and thus may have historical significance. A few references related to CSentry have been deleted ensuring that the existing Java-based Flyway script is not modified.

Closes CE-2437