Use TClass::GetClass(), not gROOT->GetClass()

This is a trivial technical change.  It is being requested for standardization, and for  code commonality with the ROOT6 7_3_X branch, as these changes have already been made there.
gROOT->GetClass(...) is obsolete.  It just calls TClass::GetClass(...). Instead, calling TClass::GetClass() directly is recommended by the ROOT team, and it saves a function call.