Avoid storing the public gateway's identity certificate

[gnarea]

Private gateways don't need the identity certificate of their public peer, but we're still storing as a "leftover" from the days when we used to encrypt cargo with that certificate (before integrating the Channel Session Protocol).

The only thing we need from that certificate is its public key, so that we can:

• Issue Cargo Delivery Authorisations (CDAs)
• Compute the public gateway's private address

[github-actions]

🎉 This issue has been resolved in version 1.5.1 🎉

The release is available on:

• v1.5.1
• GitHub release

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