Update:You can add custom css rules to your page in your admin page by following: Appearance -> Customize -> CSS (but only if you are a premium member)
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I am afraid there is no easy solution. <?php get_sidebar(); ?> is just generating content for you sidebar, the actual sidebar is showing up because of the css rules (for example, if you use “twentyfifteen” theme its in /wp-content/themes/twentyfifteen/style.css). It is adding “::before” to body element (which to my mind is very annoying). So just find “body:before” (there is usually multiple) in your css and remove it.