Consider removing compatibility code from Neo's registerCandidate

[AnnaShaleva]

Is your feature request related to a problem? Please describe.

#3700 introduces a workaround to native Neo's registerCandidate callback to ensure state processing compatibility before Echidna:

neo-go/pkg/core/native/native_neo.go

Lines 827 to 834 in e80e08d

	if !ic.IsHardforkEnabled(config.HFEchidna) {
	ok, err := runtime.CheckKeyedWitness(ic, pub)
	if err != nil {
	panic(err)
	} else if !ok {
	return stackitem.NewBool(false)
	}
	}

See also neo-project/neo#3597 (comment).

This compatibility code may be dropped after transition to Echidna, but we need to check public networks for compatibility before this change.

Describe the solution you'd like

Drop the code, process N3 mainnet/testnet, ensure states are not affected by this change.