refiller: remove needless has_connections() method

This is just a negation of `is_empty()` method.

For reference:
```
    fn is_empty(&self) -> bool {
        self.conns.iter().all(|conns| conns.is_empty())
    }
```

scylla/src/transport/connection_pool.rs

@@ -988,7 +988,7 @@ impl PoolRefiller {
     // `last_error` must not be `None` if there is a possibility of the pool
     // being empty.
     fn update_shared_conns(&mut self, last_error: Option<ConnectionError>) {
-        let new_conns = if !self.has_connections() {
+        let new_conns = if self.is_empty() {
             Arc::new(MaybePoolConnections::Broken(last_error.unwrap()))
         } else {
             let new_conns = if let Some(sharder) = self.sharder.as_ref() {
@@ -1046,7 +1046,7 @@ impl PoolRefiller {
                 self.conns[shard_id].len(),
                 self.active_connection_count(),
             );
-            if !self.has_connections() {
+            if self.is_empty() {
                 let _ = self.pool_empty_notifier.send(());
             }
             self.update_shared_conns(Some(last_error));
@@ -1152,10 +1152,6 @@ impl PoolRefiller {
         );
     }
 
-    fn has_connections(&self) -> bool {
-        self.conns.iter().any(|v| !v.is_empty())
-    }
-
     fn active_connection_count(&self) -> usize {
         self.conns.iter().map(Vec::len).sum::<usize>()
     }