bugfix in the StsSetup

This MR fixes the following compiler warning:

/home/cbmdock/cbmroot/core/detectors/sts/CbmStsSetup.cxx: In member function 'void CbmStsSetup::RecomputePhysicalAssmbBbox(TGeoManager*)':
/home/cbmdock/cbmroot/core/detectors/sts/CbmStsSetup.cxx:382:59: warning: unused parameter 'geo' [-Wunused-parameter]
 void CbmStsSetup::RecomputePhysicalAssmbBbox(TGeoManager* geo)
                                              ~~~~~~~~~~~~~^~~

The problem is that RecomputePhysicalAssmbBbox() method uses gGeoManager instead of the input variable "geo". It doesn't matter much because, in the calling code, the "geo" variable is always initialized with gGeoManager. But for consistency, one should either remove the "geo" input variable and always use gGeoManager, or use the variable. I don't know which way is better. I rewrote the code so that it uses the variable.