How can I remove data-target=”#” from bootstrap dropdown menu?

Your options are the following:

1. Extend the walker and change the output (override start_el function)

Here is an example of the Walker, assuming you are using this bootstrap walker.

Usage:

  'walker' => new se150102_bootstrap_walker()

Walker:

class se150102_bootstrap_walker extends wp_bootstrap_navwalker  {
/**
     * This walker is identical to the parent start_el() function, except
     * it does not put href="#" for parent elements.
     *
 * @see wp_bootstrap_navwalker::start_el()
 * @since 3.0.0
 *
 * @param string $output Passed by reference. Used to append additional content.
 * @param object $item Menu item data object.
 * @param int $depth Depth of menu item. Used for padding.
 * @param int $current_page Menu item ID.
 * @param object $args
 */
public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
    $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

    /**
     * Dividers, Headers or Disabled
     * =============================
     * Determine whether the item is a Divider, Header, Disabled or regular
     * menu item. To prevent errors we use the strcasecmp() function to so a
     * comparison that is not case sensitive. The strcasecmp() function returns
     * a 0 if the strings are equal.
     */
    if ( strcasecmp( $item->attr_title, 'divider' ) == 0 && $depth === 1 ) {
        $output .= $indent . '<li role="presentation" class="divider">';
    } else if ( strcasecmp( $item->title, 'divider') == 0 && $depth === 1 ) {
        $output .= $indent . '<li role="presentation" class="divider">';
    } else if ( strcasecmp( $item->attr_title, 'dropdown-header') == 0 && $depth === 1 ) {
        $output .= $indent . '<li role="presentation" class="dropdown-header">' . esc_attr( $item->title );
    } else if ( strcasecmp($item->attr_title, 'disabled' ) == 0 ) {
        $output .= $indent . '<li role="presentation" class="disabled"><a href="#">' . esc_attr( $item->title ) . '</a>';
    } else {

        $class_names = $value="";

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;
        $classes[] = 'menu-item-' . $item->ID;

        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );

        if ( $args->has_children )
            $class_names .= ' dropdown';

        if ( in_array( 'current-menu-item', $classes ) )
            $class_names .= ' active';

        $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

        $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
        $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

        $output .= $indent . '<li' . $id . $value . $class_names .'>';

        $atts = array();
        $atts['title']  = ! empty( $item->title )   ? $item->title  : '';
        $atts['target'] = ! empty( $item->target )  ? $item->target : '';
        $atts['rel']    = ! empty( $item->xfn )     ? $item->xfn    : '';

        // If item has_children add atts to a.
        if ( $args->has_children && $depth === 0 ) {
            $atts['href']           = ! empty( $item->url ) ? $item->url : '#';
            $atts['data-toggle']    = 'dropdown';
            $atts['class']          = 'dropdown-toggle';
            $atts['aria-haspopup']  = 'true';
        } else {
            $atts['href'] = ! empty( $item->url ) ? $item->url : '';
        }

        $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args );

        $attributes="";
        foreach ( $atts as $attr => $value ) {
            if ( ! empty( $value ) ) {
                $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
                $attributes .= ' ' . $attr . '="' . $value . '"';
            }
        }

        $item_output = $args->before;

        /*
         * Glyphicons
         * ===========
         * Since the the menu item is NOT a Divider or Header we check the see
         * if there is a value in the attr_title property. If the attr_title
         * property is NOT null we apply it as the class name for the glyphicon.
         */
        if ( ! empty( $item->attr_title ) )
            $item_output .= '<a'. $attributes .'><span class="glyphicon ' . esc_attr( $item->attr_title ) . '"></span>&nbsp;';
        else
            $item_output .= '<a'. $attributes .'>';

        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= ( $args->has_children && 0 === $depth ) ? ' <span class="caret"></span></a>' : '</a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
    }
}

2. Use JavaScript.

Example here in jQuery

$('#main-menu a').each(function(){
    $(this).removeAttr('data-target');
});