Remove unused m_debug_only member from Arg struct

Summary:
This is a partial backport of Core PR16097 : https://github.com/bitcoin/bitcoin/pull/16097/commits/9a12733508e47f558959f1b0ed9937bc3eff8962

Depends on D5768

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5769