Resolved an interference between Geant4 and Boost.

As I found on

https://lists.boost.org/boost-users/2016/04/86022.php

, some versions of Geant4 and Boost can end up catastrophically interfering with each
other. This super fragile #undef statement is there to work around one instance
of this happening in the ATLAS offline code.

The issue comes from G4 and Boost both using the 'times' definition, but in different
ways...