refactor(iroh-net): remove unused expired field from Endpoint

[divagant-martian]

Description

This field is unused and models a concept from tailscale that we don't have. This doesn't mean the peer's info is stale, or that it was last seen long ago, it means that the peer's public key expired based on an explicit KeyExpiry *time.Time.

From their docs

// Expired is whether this node's key has expired. Control may send
// this; clients are only allowed to set this from false to true. On
// the client, this is calculated client-side based on a timestamp sent
// from control, to avoid clock skew issues.

In tailscale the Expired is only set by a ExpiryManager based on that timestamp, from local computation; and only when the NetworkMap has not updated it based on a MapResponse requested to the server. We have none of this concepts.

If we want to define that a peer/endpoint expired, we should do it on our terms later on if the need arises.

Notes & open questions

none

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.