Remove build artifiacts from the g4 build

[gordonwatts]

Looking inside the GEANT4 build as a result of the docker file we see /root/GEANT4 is sitting at 1.7 GB - but a lot of that is (I assume) .o files and the like - which could probably be deleted (??).

The install procedure seems to put g4 executables in all sorts of other places.

root@ba9f67280bd2:~# du -h --max-depth=1
1.7G    ./GEANT4
1.7G    .
root@ba9f67280bd2:~#