In a Google+ post, Google announced that they have decided to remove the “Demote Sitelink” setting in Search Console.
Google said, they are simplifying things since the algorithms have gotten better at finding, creating , and showing relevant sitelinks.
“We only show sitelinks for results when we think they'll be useful to the user. If the structure of your site doesn't allow our algorithms to find good sitelinks, or we don't think that the sitelinks for your site are relevant for the user's query, we won't show them. This process is completely automated. Sitelinks have evolved into being based on traditional web ranking, so the way to influence them is the same as other web pages.”
This is how sitelinks look like:
Best practices to improve the quality of your sitelinks include:
# Provide a clear structure for your website, using relevant internal links and anchor text that's informative, compact, and avoids repetition.
# Allow Google to crawl and index important pages within your site. Use Fetch and Render  to check that they can be rendered properly.
# If you need to remove a page from search completely, use a "noindex" robots meta tag  on that page.