Remove legacy remote node administration

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Firmware 2.5 introduced Public Key Cryptography (PKC) to Meshtastic. With it came the ability for node operators to specify which keys should be allowed to remotely administer a node. This was a major security improvement - previously node operators had to have a single management channel and one pre-shared key to secure their nodes.

Until now, we've kept the 'legacy' functionality in place, so users of version 2.5+ firmware could still manage older nodes. Now firmware 2.5+ is in widespread use, and the new remote node administration functionality is well-tested, we can safely remove 'legacy' remote node administration. This has benefits of a smaller and more maintainable code-base, reducing a potential vulnerability vector.

References: https://meshtastic.org/docs/overview/encryption/ https://meshtastic.org/docs/development/reference/encryption-technical/ https://meshtastic.org/docs/configuration/remote-admin/