Remove obsolete _translationstatus.txt files

[Wiktor Walc]

I do not think it still makes a lot of sense to keep the _translationstatus.txt text files in CKEditor lang folders.

A much better overview of current translation status (with up-to-date stats) can be checked online @ ​CKEditor project page @ Transifex

We still can have a text file in the lang folder (e.g. readme.md), but with an explanation how one can contribute and with a direct link to CKEditor UI translation center, plus eventually a link to documentation about how one can contribute.

I guess providing such a file is not a requirement though.