[backport#15139] util: remove unused [U](BEGIN|END) macros

Summary:
https://github.com/bitcoin/bitcoin/pull/15139/commits/edb5bb35000e13e001e2693fbff4e8cc9358e166

Partial backport of Core PR15139

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6292