Reduce the use of ClearArg and only guarantee that we clear forced args.
Summary: This feature doesn't exist in Core, so reducing its surface make sense. It is not strictly limited to forced args at the moment, but the name provide intent and we'll get there after backporting Core PR15934 .
Test Plan:
make check
Updated test to reflect the new behavior.
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D5850