Take CChainParams explicitely in PaymentServer

Summary:
Take CChainParams instead of a prefix. This loses the legacy address support, but allow for further simplification.

Depends on D3539

Test Plan:
Build and run btcoin-qt
Use the open URI feature and ensure it still works.

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3548