Stop legacy spork P2P messaging on mainnet now that sporks are hardened #7874

Mainnet sporks are hardcoded (spork.cpp 'Harden all sporks on Mainnet'), but nodes still request, serve, and relay SPORK/GETSPORKS on mainnet.
Proposal: on mainnet skip GETSPORKS at sync, ignore SPORK/GETSPORKS, treat MSG_SPORK inv as already-have, and don't relay. Keep current behavior on test networks.
Prior draft: knst#3 (follow-up to #7303).

Follow-up from knst#3 (closed as stale).

🤖 Posted autonomously by Claude on behalf of pasta.