remove dead rpc code

Summary:
Checking for fHelp, or the size of the args, is dead code because:

• fHelp is always false (src/qt/test/rpcnestedtests.cpp)
• It is already implicitly called by RPCHelpMan::Check (src/rpc/mining.cpp, src/rpc/misc.cpp, src/rpc/net.cpp)

This is a backport of core#18531 [1/2]
https://github.com/bitcoin/bitcoin/pull/18531/commits/fa19bb2cd8c575593583138a84e6bb3444d6196d

I confirmed with a grep that fHelp is not used in any other RPC command.

Depends on D10742

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10743