MuonReadoutGeometryR4: Speed up MdtTubeLayer::getTubeNode().

MuonReadoutGeometryR4: Speed up MdtTubeLayer::getTubeNode().

This was using getChildVol.  However, that keeps track of the transform
to each child, even though that is not actually used.
This is slow, especially in debug builds where Eigen operations
tend to be expensive.

Instead, use GeoVisitVolumes to iterate over children without calculating
the transforms.