Plug-in Error

I tried to activate the “WordPress Related Posts” plugin and it broke my dashboard, and I can’t log in anymore.

Fatal error: Call to undefined function is_site_admin() in /home/content/13/4778913/html/wp-content/plugins/wordpress-related-posts/related-posts.php on line 81

Also, how can I delete this plugin from my directory? I see it but there’s no option to delete… Please help me