refactor: [torrust#1495] update method Swarm::meets_retaining_policy

Changed from:

```
/// Returns true if the torrents meets the retention policy, meaning that
/// it should be kept in the tracker.
pub fn meets_retaining_policy(&self, policy: &TrackerPolicy) -> bool {
    if policy.persistent_torrent_completed_stat && self.metadata().downloaded > 0 {
        return true;
    }

    if policy.remove_peerless_torrents && self.is_empty() {
        return false;
    }

    true
}
```

To:

```
pub fn meets_retaining_policy(&self, policy: &TrackerPolicy) -> bool {
    !(policy.remove_peerless_torrents && self.is_empty())
}
```

I think the first condition was introduced to avoid loosing the number of
downloads we¡hen the torrent is removed becuase there are no peers.

Now, we load that number from database when the torrent is added again
after removing it from the tracker.

Author: josecelano

packages/torrent-repository/src/swarm.rs

@@ -190,20 +190,11 @@ impl Swarm {
         self.peers.is_empty()
     }
 
-    /// Returns true if the torrents meets the retention policy, meaning that
+    /// Returns true if the swarm meets the retention policy, meaning that
     /// it should be kept in the tracker.
     #[must_use]
     pub fn meets_retaining_policy(&self, policy: &TrackerPolicy) -> bool {
-        // code-review: why?
-        if policy.persistent_torrent_completed_stat && self.metadata().downloaded > 0 {
-            return true;
-        }
-
-        if policy.remove_peerless_torrents && self.is_empty() {
-            return false;
-        }
-
-        true
+        !(policy.remove_peerless_torrents && self.is_empty())
     }
 }
 
@@ -214,6 +205,7 @@ mod tests {
     use std::sync::Arc;
 
     use aquatic_udp_protocol::PeerId;
+    use torrust_tracker_configuration::TrackerPolicy;
     use torrust_tracker_primitives::peer::fixture::PeerBuilder;
     use torrust_tracker_primitives::swarm_metadata::SwarmMetadata;
     use torrust_tracker_primitives::DurationSinceUnixEpoch;
@@ -384,6 +376,30 @@ mod tests {
         assert_eq!(swarm.len(), 1);
     }
 
+    #[test]
+    fn it_should_be_kept_when_empty_if_the_tracker_policy_is_not_to_remove_peerless_torrents() {
+        let empty_swarm = Swarm::default();
+
+        let policy = TrackerPolicy {
+            remove_peerless_torrents: false,
+            ..Default::default()
+        };
+
+        assert!(empty_swarm.meets_retaining_policy(&policy));
+    }
+
+    #[test]
+    fn it_should_be_removed_when_empty_if_the_tracker_policy_is_to_remove_peerless_torrents() {
+        let empty_swarm = Swarm::default();
+
+        let policy = TrackerPolicy {
+            remove_peerless_torrents: true,
+            ..Default::default()
+        };
+
+        assert!(!empty_swarm.meets_retaining_policy(&policy));
+    }
+
     #[test]
     fn it_should_allow_inserting_two_identical_peers_except_for_the_socket_address() {
         let mut swarm = Swarm::default();