Remove unused raw-pointer read helper from univalue
Summary:
The helpers are unused outside of tests and redundant with the existing bool read(std::string_view raw);. Fix both issues by removing them. Also, simplify the tests code by removing a std::string constructor where possible.
Backport of core#28168.
Depends on D16295.
Test Plan:
ninja all check
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D16296