Source

File: wp-includes/nav-menu.php

function unregister_nav_menu( $location ) {
	global $_wp_registered_nav_menus;

	if ( is_array( $_wp_registered_nav_menus ) && isset( $_wp_registered_nav_menus[ $location ] ) ) {
		unset( $_wp_registered_nav_menus[ $location ] );
		if ( empty( $_wp_registered_nav_menus ) ) {
			_remove_theme_support( 'menus' );
		}
		return true;
	}
	return false;
}