Make all CRPCCommand actors take a const Config &

Summary:
After D9745 there seems to no longer be a need for RPC commands that
take a non-const Config as a parameter.
In D10007 and D10010, it seemed that the help RPC still needed a
non-const Config &, but it was actually no longer the case.
The help executes the other RPCs to get their help message, and since
D9745 removed setexcessiveblock, this is no longer needed.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10149