caption shortcode using image alt as caption

To remove caption text, you can override the caption short-code using below code:

add_filter( 'img_caption_shortcode', 'my_img_caption_shortcode', 10, 3 );

function my_img_caption_shortcode( $empty, $attr, $content ){
    $attr = shortcode_atts( array(
        'id'      => '',
        'align'   => 'alignnone',
        'width'   => '',
        'caption' => ''
    ), $attr );

    if ( 1 > (int) $attr['width'] || empty( $attr['caption'] ) ) {
        return '';
    }

    if ( $attr['id'] ) {
        $attr['id'] = 'id="' . esc_attr( $attr['id'] ) . '" ';
    }

    return '<div ' . $attr['id']
    . 'class="wp-caption ' . esc_attr( $attr['align'] ) . '" '
    . 'style="max-width: ' . ( 10 + (int) $attr['width'] ) . 'px;">'
    . do_shortcode( $content )
    . '</div>';

}

For more details, refer CODEX

UPDATE:

Try changing below mentioned line.

add_filter( 'img_caption_shortcode', 'my_img_caption_shortcode', 999, 3 );

Hope this helps!