[ENH] Remove external database support from persistence service example in community

[fercsrti77]

Enhancement description

In Connext 7.0, we do not support external database support in Persistence Service, durable writer history, and durable reader state. This issue removes external database support from the persistence_service example.

Because the only way to use durable writer history and durable reader is with an external database, this issue also removes the testing of these features from the example.