return unformatted content in encosing shortcode

The <br /> tags are being added to the content by the wpautop() function. If you run your shortcode before it runs they will not be there. It is added in \wp-includes\default-filters.php with the default priority of 10.

add_filter( 'the_content', 'myshortcode', 1 );

function myshortcode( $atts, $content = null ) {
    return $content;
}

wpautop() can also add other markup besides <br /> tags.