add_filter( 'post_thumbnail_html', 'remove_thumbnail_dimensions', 10 );
add_filter( 'image_send_to_editor', 'remove_thumbnail_dimensions', 10 );
// Removes attached image sizes as well
add_filter( 'the_content', 'remove_thumbnail_dimensions', 10 );
function remove_thumbnail_dimensions( $html ) {
    $html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
    return $html;
}