Issue #1968322 by effulgentsia: Remove unused $id and $zebra variables from templates.

core/includes/theme.inc

core/modules/block/block.module

core/modules/block/templates/block.tpl.php

core/modules/node/templates/node.html.twig

core/modules/node/templates/node.tpl.php

core/modules/taxonomy/templates/taxonomy-term.tpl.php

core/themes/bartik/templates/node.tpl.php