Variable(node) {
            const varName = node.name.value;
            const varDef = variableDefinitionByName(varName);

            subVariableUsageCountByName[varName] =
              subVariableUsageCountByName[varName] + 1 || 1;

            if (!varDef) {
              return;
            }

            return varDef.defaultValue;
          }
In the process of devariabilizing descendent selections,
we may have caused their variable definitions to become unused.
Keep track and remove any variable definitions with 1 or fewer usages.