libstore: remove curlFileTransfer::enqueueItem return type

it's used only once, and the transfer is used for something else
afterwards as well. returning it hides that the same transfer is
being returned unchanged, and pointlessly extends the signature.

Change-Id: Idbd0af00f9aebd3606f6b537b9b7cc9263a45cf7

src/libstore/filetransfer.cc

@@ -712,7 +712,7 @@ struct curlFileTransfer : public FileTransfer
         }
     }
 
-    std::shared_ptr<TransferItem> enqueueItem(std::shared_ptr<TransferItem> item)
+    void enqueueItem(std::shared_ptr<TransferItem> item)
     {
         if (item->uploadData
             && !item->uri.starts_with("http://")
@@ -726,7 +726,6 @@ struct curlFileTransfer : public FileTransfer
             state->incoming.push(item);
         }
         wakeup();
-        return item;
     }
 
 #if ENABLE_S3
@@ -857,7 +856,7 @@ struct curlFileTransfer : public FileTransfer
 
         auto _state = std::make_shared<Sync<State>>();
 
-        auto item = enqueueItem(std::make_shared<TransferItem>(
+        auto item = std::make_shared<TransferItem>(
             *this,
             uri,
             headers,
@@ -888,7 +887,8 @@ struct curlFileTransfer : public FileTransfer
             },
             std::move(data),
             noBody
-        ));
+        );
+        enqueueItem(item);
 
         struct TransferSource : Source
         {