G4UI_USE and G4VIS_USE removed in Geant4 10.6, breaks compilation

[a2wagner]

The preprocessor macros G4UI_USE and G4VIS_USE are removed with the release of Geant4 version 10.6, see release notes. They are currently used in the beginning of A2.cc and are essential for A2.cc#L167 to compile. By removing the #ifdef preprocessor macros in the beginning of the file the code compiles again.

Further checks are required to assure the code still works as intended, otherwise we should think about specifically mentioning that 10.6 is not supported. (Implicitly this is the case right now as in the dependencies section it is not mentioned)