refactor: Remove g_rpc_node global

Summary:

This commit does not change behavior

Concludes backport of core PR18740:
https://github.com/bitcoin/bitcoin/pull/18740/commits/b3f7f375efb9a9ca9a7a4f2caf41fe3df2262520

Depends on D7991.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7992