Removed TrackPropagation/Geant4e from Threaded branch

The Geant4e based propagator is not presently thread safe. To avoid
problems with the static analyzer we are removing it from the threaded
branch. If the code is made thread safe, we will add it back.
Merge pull request #3190 from Dr15Jones/removeTrackPropagationGeant4eFromThreaded

Multithreading -- Removed TrackPropagation/Geant4e from Threaded branch