Loading libCore.so pyHepMC3/rootIO/_init_.py messes up MacOS

Loading explicitly libCore.so in pyHepMC3/rootIO/_init_.py is not need since pyHepMC3rootIO.so has a direct dependency to libCore.so. Doing it messes up MacOS and generates an error.

See the following issue https://sft.its.cern.ch/jira/browse/SPI-2179

The solution is to remove these lines.