Revert "[Serialization] (De-)serialize the substitutions of value witnesses."

This reverts commit 9b583cdb84.

lib/Serialization/Serialization.cpp

@@ -1053,8 +1053,6 @@ void Serializer::writeNormalConformance(
     data.push_back(addDeclRef(witness.getDecl()));
     assert(witness.getDecl() || req->getAttrs().hasAttribute<OptionalAttr>()
            || req->getAttrs().isUnavailable(req->getASTContext()));
-    // The substitution record is serialized later.
-    data.push_back(witness.getSubstitutions().size());
     ++numValueWitnesses;
   });
 
@@ -1101,12 +1099,6 @@ void Serializer::writeNormalConformance(
                      DeclTypeAbbrCodes);
   }
 
-  conformance->forEachValueWitness(nullptr,
-                                   [&](ValueDecl *req,
-                                       ConcreteDeclRef witness) {
-    writeSubstitutions(witness.getSubstitutions(), DeclTypeAbbrCodes);
-    return false;
-  });
   conformance->forEachTypeWitness(/*resolver=*/nullptr,
                                   [&](AssociatedTypeDecl *assocType,
                                       const Substitution &witness,