Cleanup unused RPCServer namespace

Summary: Cleanup unused namespace to reduce future namespace collisions. Split from a previous review: https://reviews.bitcoinabc.org/D739

Test Plan: Tests pass.

Reviewers: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D749