[Futurev4] Remove inconsistent G4MULTITHREADED definitions

Since we are at Geant4 10.6 already, manually setting the G4MULTITHREADED macro is no longer necessary (it is set when building Geant4 and propagated as a generated header (G4GlobalConfig.hh).

The inconsistency in protecting the add_definitions with the Geant4 version check meant that if Geant4 is compiled without MT enabled, the linking here fails as the headers are inconsistent (thread-local vs non-thread-local variables).