do not delete a previously existing TF1 whena a new TF1 is created with the same name.

Just remove the function from the global list (gROOT->GetListOfFunctions() )

This fixes a problem reported https://savannah.cern.ch/bugs/?69073 and 
http://root.cern.ch/phpBB3//viewtopic.php?t=10065


git-svn-id: http://root.cern.ch/svn/root/trunk@34047 27541ba8-7e3a-0410-8455-c3a389f83636