DAP-13: drop support for Poplar1

[branlwyd]

With the removal of repeated collection of a batch, Poplar1 is no longer of practical value. We should stop supporting it in the DAP-13 / 0.8 release track of Janus.

Specifically:

• Application uses of Poplar1 should be dropped entirely.
• Uses of Poplar1 in unit tests should be replaced by libprio-rs' dummy VDAF, which can be configured to require multiple rounds for completion.
• Uses of Poplar1 in integration/end-to-end tests should be dropped entirely.