Remove HfHelper instance and use static methods directly

As pointed out in code review, HfHelper methods are static and don't
require an instance. This change:

1. Removes HfHelper instance variable from both taskD0 and taskDplus
2. Changes all method calls from `hfHelper.method()` to `HfHelper::method()`

This is more efficient as it avoids unnecessary object instantiation
for methods that don't need instance state.

Changes:
- taskD0.cxx: Remove hfHelper instance, update 5 method calls
- taskDplus.cxx: Remove hfHelper instance, update 2 method calls

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: minjungkim12

PWGHF/D2H/Tasks/taskD0.cxx

@@ -105,7 +105,6 @@ struct HfTaskD0 {
 
   HfEventSelection hfEvSel; // event selection and monitoring
 
-  HfHelper hfHelper;
   ctpRateFetcher mRateFetcher;
 
   SliceCache cache;
@@ -604,12 +603,12 @@ struct HfTaskD0 {
           if (!(candidate.hfflag() & 1 << aod::hf_cand_2prong::DecayType::D0ToPiK)) {
             continue;
           }
-          if (yCandRecoMax >= 0. && std::abs(hfHelper.yD0(candidate)) > yCandRecoMax) {
+          if (yCandRecoMax >= 0. && std::abs(HfHelper::yD0(candidate)) > yCandRecoMax) {
             continue;
           }
 
-          float massD0 = hfHelper.invMassD0ToPiK(candidate);
-          float massD0bar = hfHelper.invMassD0barToKPi(candidate);
+          float massD0 = HfHelper::invMassD0ToPiK(candidate);
+          float massD0bar = HfHelper::invMassD0barToKPi(candidate);
           auto ptCandidate = candidate.pt();
 
           if (candidate.isSelD0() >= selectionFlagD0) {
@@ -628,7 +627,7 @@ struct HfTaskD0 {
             auto fillTHnData = [&](float mass, int d0Type) {
               std::vector<double> valuesToFill{static_cast<double>(mass), static_cast<double>(ptCandidate),
                                                candidate.mlProbD0()[0], candidate.mlProbD0()[1], candidate.mlProbD0()[2],
-                                               static_cast<double>(hfHelper.yD0(candidate)), static_cast<double>(d0Type)};
+                                               static_cast<double>(HfHelper::yD0(candidate)), static_cast<double>(d0Type)};
               if (storeCentrality) {
                 valuesToFill.push_back(centrality);
               }
@@ -652,7 +651,7 @@ struct HfTaskD0 {
           } else {
             auto fillTHnData = [&](float mass, int d0Type) {
               std::vector<double> valuesToFill{static_cast<double>(mass), static_cast<double>(ptCandidate),
-                                               static_cast<double>(hfHelper.yD0(candidate)), static_cast<double>(d0Type)};
+                                               static_cast<double>(HfHelper::yD0(candidate)), static_cast<double>(d0Type)};
               if (storeCentrality) {
                 valuesToFill.push_back(centrality);
               }

PWGHF/D2H/Tasks/taskDplus.cxx

@@ -91,7 +91,6 @@ struct HfTaskDplus {
 
   HfEventSelection hfEvSel; // event selection and monitoring
 
-  HfHelper hfHelper;
   Service<o2::ccdb::BasicCCDBManager> ccdb;
 
   using CandDplusData = soa::Filtered<soa::Join<aod::HfCand3Prong, aod::HfSelDplusToPiKPi>>;
@@ -735,7 +734,7 @@ struct HfTaskDplus {
         int const gapTypeInt = hf_upc::gapTypeToInt(gap);
 
         for (const auto& candidate : groupedDplusCandidates) {
-          if ((yCandRecoMax >= 0. && std::abs(hfHelper.yDplus(candidate)) > yCandRecoMax)) {
+          if ((yCandRecoMax >= 0. && std::abs(HfHelper::yDplus(candidate)) > yCandRecoMax)) {
             continue;
           }
           fillHisto(candidate);
@@ -746,7 +745,7 @@ struct HfTaskDplus {
               outputMl[iclass] = candidate.mlProbDplusToPiKPi()[classMl->at(iclass)];
             }
 
-            std::vector<double> valuesToFill{hfHelper.invMassDplusToPiKPi(candidate), static_cast<double>(candidate.pt()), outputMl[0], outputMl[1], outputMl[2]};
+            std::vector<double> valuesToFill{HfHelper::invMassDplusToPiKPi(candidate), static_cast<double>(candidate.pt()), outputMl[0], outputMl[1], outputMl[2]};
             if (storeCentrality) {
               valuesToFill.push_back(cent);
             }