Implement function Track::Clear that deletes the array fPointvalue.

The Track destructor calls the new Clear function.

In Event::Clear, replaces fTracks->Delete() by fTracks->Clear("C")
to take advantage of the new option in TClonesArray::Clear.


git-svn-id: http://root.cern.ch/svn/root/trunk@5161 27541ba8-7e3a-0410-8455-c3a389f83636