Fix build warnings (#6235)

* un/signed comparisons in molops

* reference in loop in EmbeddedFrag.cpp

* un/signed comparison in filtercatalogtest.cpp

* un/signed comparison in Descriptors/catch_tests.cpp

* un/signed comparisons in DetermineBonds.cpp

* drop unused 'params' arg from static functions in MolInterchange/Writer.cpp

* un/signed comparisons in MolDraw2D/catch_tests.cpp

* unused var in SubgraphUtils.cpp

* 'move' preventing copy ellision in CDXMLParser.cpp

* fix infinite recursive call in SubstructLibraryWrap.cpp

Code/GraphMol/Subgraphs/SubgraphUtils.cpp

@@ -135,7 +135,6 @@ DiscrimTuple calcPathDiscriminators(const ROMol &mol, const PATH_TYPE &path,
     CHECK_INVARIANT(extraInvars->size() == mol.getNumAtoms(),
                     "bad extra invars");
   }
-  DiscrimTuple res;
 
   // Start by collecting the atoms in the path and their degrees
   std::vector<int32_t> atomsUsed(mol.getNumAtoms(),