fix: [torrust#78] remove unused struct NewTrackerKey

It was added because the tracker API was changed, but that change was
reverted here:

torrust/torrust-tracker#109

and it is not used anymore.

src/models/tracker_key.rs

@@ -7,12 +7,6 @@ pub struct TrackerKey {
     pub valid_until: i64,
 }
 
-#[derive(Debug, Serialize, Deserialize)]
-pub struct NewTrackerKey {
-    pub key: String,
-    pub valid_until: Duration,
-}
-
 #[derive(Debug, Serialize, Deserialize)]
 pub struct Duration {
     pub secs: i64,