[backport#16117] util: Remove unused MilliSleep

Summary:
https://github.com/bitcoin/bitcoin/pull/16117/commits/fae86c38bca5c960462e53975314a0749db5d17d

Depends on D6518

Concludes backport of Core PR16117

Test Plan:

ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6519