Remove link to a translation that no longer exists

Connects to #375

src/appendix-06-translation.md

@@ -7,7 +7,6 @@ For resources in languages other than English. Most are still in progress; see
 
 - [Português](https://github.com/rust-br/rust-book-pt-br) (BR)
 - [Português](https://github.com/nunojesus/rust-book-pt-pt) (PT)
-- [Tiếng việt](https://github.com/hngnaig/rust-lang-book/tree/vi-VN)
 - [简体中文](https://github.com/KaiserY/trpl-zh-cn)
 - [Українська](https://github.com/pavloslav/rust-book-uk-ua)
 - [Español](https://github.com/thecodix/book), [alternate](https://github.com/ManRR/rust-book-es)