Remove ClassImp() as it is not necessary anymore but creates compiler problems

Issue with zero-as-null-pointer in LLVM5.0, not necessary according to https://stackoverflow.com/questions/38438648/classimp-preprocessor-macro-in-root-is-it-really-needed https://root-forum.cern.ch/t/issue-with-classimp-when-compiling-with-clang-5-0/27083