[qa] util: Remove unused sync_chain

Summary:

The util function sync_blocks already checks for equal chains, so we can
remove the unused sync_chain.

Backport of core PR12643
https://github.com/bitcoin/bitcoin/pull/12643/files

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3257