MuonGeoModel - Do not add the auxillary GeoModelTrees to the world volume

In the current MuonGeoModel, the readout geometry is rebuilt every time when the alignment IOV boundaries are crossed. Usually, that's happening once in a job or so. However, the auxillary GeoModel tree is appended to the root volume node of ATLAS. That's causing a problem if the input data is kind of shuffled as it's in the Mdt calibration stream and the geometry is rebuilt again and again. This patch completely disconnects the auxillary GeoModel tree from the ATLAS world volume and should hence ensure that the auxillary GeoModel tree is deleted once the alignment IOVs go out of scope. The useage of PVLink already assumes the smart pointer useage which is coming with !68268 (merged).

Tagging: @diehl, @zyan, @pscholer