Clean up unmodified functions in Electra LC

[ensi321]

Since LightClientHeader is not modified in Electra, it is safe to remove block_to_light_client_header and is_valid_light_client_header from Electra spec.

One debatable point is whether we want to keep full-node.md for Electra as there is no change to it.

[jtraglia]

Seems reasonable to me. We should remove full-node.md if there are no changes. Don't forget to remove the full node link from list of spec changes on the README, here:

consensus-specs/README.md

Line 30 in 96b1d31

| 5 | **Electra** | TBD | <ul><li>Core</li><ul><li>[Beacon Chain changes](specs/electra/beacon-chain.md)</li><li>[Electra fork](specs/electra/fork.md)</li></ul><li>Additions</li><ul><li>[Light client sync protocol changes](specs/electra/light-client/sync-protocol.md) ([fork](specs/electra/light-client/fork.md), [full node](specs/electra/light-client/full-node.md), [networking](specs/electra/light-client/p2p-interface.md))</li><li>[Honest validator guide changes](specs/electra/validator.md)</li><li>[P2P networking](specs/electra/p2p-interface.md)</li></ul></ul> |