remove unused class util::Ref and its unit test

Summary:
This concludes backport of core#21366
https://github.com/bitcoin/bitcoin/pull/21366/commits/916ab0195d567fd0a9097045e73a6654c453adea

Depends on D11551

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11552