[test] Cleanup unnecessary sync_all in rpc_net.py

Summary:
This was missed in D12535

Partial backport of core#23474

Depends on D17529

Test Plan:

./test/functional/test_runner.py rpc_net

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17539