Fix unused variables, parameters, values

Fixes errors from the following Clang-Tidy categories:

- `clang-analyzer-deadcode.DeadStores`
- `clang-diagnostic-unused-parameter`
- `clang-diagnostic-unused-variable`

Most errors are fixed temporarily by commenting out the unused bits and by adding a `FIXME: unused` comment.

The final fix is to either remove the unused bits or to fix the bugs which has to be done by someone who knows the intended behaviour.

Author: vkucera

PWGEM/Dilepton/TableProducer/treeCreatorElectronML.cxx

@@ -774,7 +774,7 @@ struct TreeCreatorElectronML {
     }
 
     fNewLabels.clear();
-    fCounter = 0;
+    // fCounter = 0; // FIXME: unused
   } // end of process
   PROCESS_SWITCH(TreeCreatorElectronML, processPair, "produce ML input for pair level", false);
 
@@ -845,7 +845,7 @@ struct TreeCreatorElectronML {
     }
 
     fNewLabels.clear();
-    fCounter = 0;
+    // fCounter = 0; // FIXME: unused
   } // end of process
   PROCESS_SWITCH(TreeCreatorElectronML, processPairSkimmed, "produce ML input for pair level on skimmed data", false);
 };

PWGEM/Dilepton/Tasks/lmeeLFCocktail.cxx

@@ -345,11 +345,11 @@ struct lmeelfcocktail {
       addHistogram1D("Ptee", ptee_axis, i);
       // addHistogram1D_stage("Dca", dca_axis, i, "rec/");
       // addHistogram1D_stage("Dcaee", dcaee_axis, i, "rec/");
-      i = -1;
+      // i = -1; // FIXME: unused
       // addHistogram2D_stage("DcaVsPt", dca_axis, pt_axis, i, "rec/");
       // addHistogram2D_stage("DcaeeVsPtee", dcaee_axis, ptee_axis, i, "rec/");
       // addHistogram2D_stage("DcaeeVsMee", dcaee_axis, mass_axis, i, "rec/");
-      i = -1;
+      i = -1; // FIXME: unused
       addHistogramND("MeeVsPteeVsCos2DPhiRP", std::vector<AxisSpec>{mass_axis, ptee_axis, cos2dphi_axis}, i);
     }
   }