fix: keep MRF channel open to avoid random CI crash (#10686)

There doesn't seem to be any benefit to closing the channel, so just keep 
it open and let it die with the server.

cmd/erasure.go

@@ -81,14 +81,6 @@ func (er erasureObjects) SetDriveCount() int {
 func (er erasureObjects) Shutdown(ctx context.Context) error {
 	// Add any object layer shutdown activities here.
 	closeStorageDisks(er.getDisks())
-	select {
-	case _, ok := <-er.mrfOpCh:
-		if ok {
-			close(er.mrfOpCh)
-		}
-	default:
-		close(er.mrfOpCh)
-	}
 	return nil
 }