univalue: Remove unused and confusing set*() return value
Summary:
The value is: - currently unused, and useless without [[nodiscard]] - confusing, because it is always true, unless a num-string is set Instead of adding [[nodiscard]], throw when setting is not possible.
Backport of core#25736.
Depends on D16283.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D16284