4670006762ffce654bb12edb5a7e64ad004122a7 test: remove redundant sync_with_ping after add_p2p_connection (Jon Atack)

Pull request description:

Now that #18247 is merged, these calls are redundant.

ACKs for top commit:

vasild:
  utACK 4670006

Tree-SHA512: bdbfe8bcf9dbdde0a8115e3a62bfe359910798d7a3010d920ffca07049cb5f97bf8fb9b6f70079b0607105192b61a6d665774e59a2b678597b47ad6237595ad5

Backport of Core PR18496

Run p2p_sendheaders a few times to make sure it's not flaky:

for X in {1..10}; do ./test/functional/test_runner.py p2p_sendheaders ; done