remove unnecessary sync_blocks in p2p_compactblocks

Summary:
This was missed in D14872

Depends on D15110

Test Plan:

for i in {1..10}; do test/functional/test_runner.py p2p_compactblocks; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15112