Generating rel=prev and rel=next only on wordpress categories

Not entirely sure if I agree with the explanation of your SEO guy as rel=prev and rel=next are used for paginated archives.

Of course categories can be one, but also your blog if it’d contain multiple pages.

Anyways, here’s what I’m currently using:

remove_action( 'wp_head', 'adjacent_posts_rel_link_wp_head' );

add_action( 'wp_head', 'cor_rel_next_prev_pagination' );
/**
 * Pagination with rel="next" and rel="prev".
 *
 * @link wp-includes|default-filters.php
 * @link http://core.trac.wordpress.org/ticket/18672 Implement rel="prev" and rel="next" for archives
 * @link http://googlewebmastercentral.blogspot.com/2011/09/pagination-with-relnext-and-relprev.html Pagination with rel="next" and rel="prev"
 */
function cor_rel_next_prev_pagination() {
  global $paged;
  if ( get_previous_posts_link() ) {
?>
  <link rel="prev" href="https://wordpress.stackexchange.com/questions/47638/<?php echo get_pagenum_link( $paged - 1 ); ?>">
<?php
  }
  if ( get_next_posts_link() ) {
?>
  <link rel="next" href="<?php echo get_pagenum_link( $paged + 1 ); ?>">
<?php
  }
}

Edit: also I believe start_post_rel_link is no longer being used.