Remove obsolete NODISCARD ifdef forest. Use nodiscard

Summary:

scripted-diff: Use nodiscard (C++17) instead of NODISCARD

-BEGIN VERIFY SCRIPT-
sed -i "s/NODISCARD/[[nodiscard]]/g" $(git grep -l "NODISCARD" ":(exclude)src/bench/nanobench.h" ":(exclude)src/attributes.h")
-END VERIFY SCRIPT-

Remove NODISCARD

This is a backport of core#20499
Depends on D10888

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10889