[RF] Remove redundante dependsOn() when creating norm. integral

The `depList` list is filled already with the subset of normalization
variables that the pdf depends on (see `getObservables(nset, depList)` a
few lines before. Therefore, the check `dependsOn(depList)` is
redundant and causes unnecessary performance overheads, as the whole
compute graph of the pdf has to be traversed.

We might as well check if `depList` is empty for the same logic.

(cherry picked from commit c1608ac)

Author: guitargeek

roofit/roofitcore/src/RooAbsPdf.cxx

@@ -532,7 +532,7 @@ bool RooAbsPdf::syncNormalization(const RooArgSet* nset, bool adjustProxies) con
   }
 
   // Destroy old normalization & create new
-  if (selfNormalized() || !dependsOn(depList)) {
+  if (selfNormalized() || depList.empty()) {
     auto ntitle = std::string(GetTitle()) + " Unit Normalization";
     auto nname = std::string(GetName()) + "_UnitNorm";
     _norm = new RooRealVar(nname.c_str(),ntitle.c_str(),1) ;