Mysterious line in top of the submenu

There is a line appearing at the top of the submenus. How can I hide it? Or make it repeated throughout the submenus?

http://imgur.com/WzeACct

Hello @aqeelat,

Please try to use the following CSS code in order to hide it.

You can add CSS code in DashboardAppearanceCustomizeAdditional CSS (WordPress 4.7 and up). Also you can add CSS code directly to style.css file of your child theme.

#mainnav ul li ul::after {
    display: none;
}

Please feel free to ask any other questions that you might have.

Kind Regards, Roman.