[RF] Remove warning about unused variables in RooFormula

After commit a27e60a, it is not important anymore that only the
variables used by the expression are passed to RooFormula.

Removing the corresponding warnings helps to get rid of useless warnings
in the case where you want to try out variations of the formula that
omit certain terms, and in particular it helps in
`RooAbsData::reduce()`, where the formula is always passed all the
varaiables in the dataset, whether the reduction uses them or not.

roofit/roofitcore/src/RooFormula.cxx

@@ -201,13 +201,6 @@ RooFormula::RooFormula(const char* name, const char* formula, const RooArgList&
   _isCategory = findCategoryServers(_origList);
 
   installFormulaOrThrow(formula);
-
-  RooArgList useList = usedVariables();
-  if (checkVariables && _origList.size() != useList.size()) {
-    coutI(InputArguments) << "The formula " << GetName() << " claims to use the variables " << _origList
-        << " but only " << useList << " seem to be in use."
-        << "\n  inputs:         " << formula << std::endl;
-  }
 }