replace io.Discard usage to fix some NUMA copy() latencies (#18394)

replace io.Discard usage to fix NUMA copy() latencies

On NUMA systems copying from 8K buffer allocated via
io.Discard leads to large latency build-up for every

```
copy(new8kbuf, largebuf)
```

can in-cur upto 1ms worth of latencies on NUMA systems
due to memory sharding across NUMA nodes.

cmd/storage-rest-client.go

@@ -856,10 +856,7 @@ func (client *storageRESTClient) CleanAbandonedData(ctx context.Context, volume
 		return err
 	}
 	defer xhttp.DrainBody(respBody)
-	respReader, err := waitForHTTPResponse(respBody)
-	if err == nil {
-		io.Copy(io.Discard, respReader)
-	}
+	_, err = waitForHTTPResponse(respBody)
 	return err
 }