TASK: Remove v1 of PQ page implementation

[yaauie]

Starting with Logstash 6.3.0 (June 2018), pipelines using queue.type: persisted have written and processed events using a "V2" implementation. When a pipeline is started and a non-empty "V1" PQ is observed on disk, the V1 PQ page files are upgraded in place before the pipeline can complete starting up.

Since we have no reasonable expectation for Logstash 9.x to load PQ page files that were left behind by Logstash < 6.3.0, we can safely remove the V1 implementation and the queue version upgrade code, simplifying the footprint of maintaining and evolving the PQ.