This field doesn't actually appear in `GuardianSetUpgrade` governance
messages and was already being skipped by serde so just remove it
completely. Contracts that need to keep track of this information can
encapsulate the `GuardianSetInfo` inside another struct that has an
`expiration_time` field.
The serde_wormhole crate implements the wormhole wire format as a serde
data format. This will let us replace all the hand-rolled
serialization with auto-generated code, which is less error-prone and
easier to review.