test: Replace gArgs with local argsman in bench

Summary:

• tools: Add unused argsman to bench_bitcoin
• scripted-diff: Replace gArgs with local argsman in bench

-BEGIN VERIFY SCRIPT-
sed -i -e 's/gArgs/argsman/g' src/bench/bench_bitcoin.cpp
-END VERIFY SCRIPT-

• bench: Remove unused argsman.ClearArgs
• util: Document why ArgsManager (con/de)structor is not inline

Backport of Core PR18662

Test Plan:

ninja bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8079