Remove Greek, Turkish, and Vietnamese community translations

[lamberta]

Hi folks,

We're changing our community translation workflow which requires new prioritization rules. See this message for some context.

As such, removing the Greek, Turkish, and Vietnamese community translations since we're unable to build a sustainable community with our available resources.

cc /site/el/REVIEWERS: @ChiefGeorge
cc /site/tr/REVIEWERS: n/a
cc /site/vi/REVIEWERS: @truongsinh @truongnh1992 @anniehiehie @duythanhvn

From the README:

Language support is determined by a number of factors including—but not limited to—site metrics and demand, community support, English proficiency, audience preference, and other indicators. Since each supported language incurs a cost, unmaintained languages are removed. Support for new languages will be announced on the TensorFlow blog or Twitter.

If your preferred language is not supported, you are welcome to maintain a community fork for open source contributors. These are not published to tensorflow.org.

Thank you everyone for all your work and support! We're making good progress with our machine translation program and will hopefully have a way for the community to review and improve the glossary.