sdk/rust: Remove `expiration_time` from `GuardianSetInfo`

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.

sdk/rust/core/src/core.rs

@@ -541,7 +541,6 @@ mod test {
                                 0xeb, 0x15, 0xe8, 0x0c, 0x9a, 0x99, 0xc8, 0x34, 0x8d,
                             ]),
                         ],
-                        expiration_time: 0,
                     },
                 },
             },

sdk/rust/core/src/lib.rs

@@ -65,10 +65,6 @@ pub struct Amount(pub [u8; 32]);
 pub struct GuardianSetInfo {
     /// The set of guardians public keys, in Ethereum's compressed format.
     pub addresses: Vec<GuardianAddress>,
-
-    /// How long after a GuardianSet change before this set is expired.
-    #[serde(skip)]
-    pub expiration_time: u64,
 }
 
 impl GuardianSetInfo {
@@ -112,7 +108,6 @@ mod test {
         for (count, quorum) in tests {
             let gs = GuardianSetInfo {
                 addresses: vec![Default::default(); count],
-                expiration_time: 0,
             };
 
             assert_eq!(quorum, gs.quorum());

sdk/rust/serde_wormhole/src/lib.rs

@@ -308,7 +308,6 @@ mod tests {
     struct GuardianSetInfo<'a> {
         #[serde(borrow)]
         addresses: Cow<'a, [GuardianAddress<'a>]>,
-        expiration_time: u64,
     }
 
     #[derive(Debug, Serialize, Deserialize, PartialEq, Eq)]