Close export results channel in generic implementation

This removes the need for every implementation to handle this case.

internal/backends/maloja/maloja.go

@@ -67,8 +67,6 @@ func (b *MalojaApiBackend) ExportListens(oldestTimestamp time.Time, results chan
 	page := 0
 	perPage := MaxItemsPerGet
 
-	defer close(results)
-
 	// We need to gather the full list of listens in order to sort them
 	listens := make(models.ListensList, 0, 2*perPage)
 	p := models.Progress{Total: int64(perPage)}