Remove config managed RPC user/pass

Summary: The user/pass is read from garg and stored into the config, to be fetched from there again by the exact same module. Even if we can move away from having so much global state, then a member variable for the http thing would make sense, not the config.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox, majcosta

Reviewed By: #bitcoin_abc, jasonbcox, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D5925