Yandex disk removed from supported WEBDAV clients (#3432)

Yandex.disk recently removed file upload support from WebDAV

https://ru.stackoverflow.com/questions/1043523/После-загрузки-файла-на-Яндекс-Диск-по-webdav-происходят-паузы-по-60-секунд-на-к
https://qna.habr.com/q/677787

So this makes Yandex.disk unusable at the moment.

Here's a similar issue, please see recent comments: https://github.com/PhilippC/keepass2android/issues/506

I've personally tried, and cannot even access to Yandex.disk at the moment through WebDAV.

This PR simply removes yandex.disk line from the ReadMe.

Thanks,