rootcling: remove libCling

libCling is loaded by dlopen in ROOT6. Once TROOT::InitInterpreter()
is called it first checks if there are no LLVM symbols available
from alternative (user) LLVM. We don't need to link to libCling,
which triggers this check. ROOT6 uses dlsym to look for
LLVMEnablePrettyStackTrace symbol before loading libCling.

For future, we probably need to get rid of rootcling toolfile.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>