/**
 * Removes inline width and height attributes for images. Thanks to
 * @boonebgorges for help with this!
 **/
function clioweb_remove_inline_sizes($html) {
    $pattern = '/(\sheight|\swidth)="(.*?)"/';
    $html = preg_replace($pattern, '', $html);
    return $html;
}

add_filter( 'get_image_tag', 'clioweb_remove_inline_sizes', 10, 1 );