QDoc: Get rid of sloppy translation effort

Our current practice add maintenance cost and burden (arguably, the maintenance burden is rather low as we don’t actually do it!). We provide a poor user experience that adds little value, or is even detrimental to perceived value, as the implementation doesn't adhere to best practices (which is typically caused by parts of the messages QDoc issue are being forwarded from sources external to QDoc itself; Clang, source files, etc). Furthermore, the business value of making QDoc translatable is negligible (at best), as we do not have requests from neither internal nor external customers.

There is a cost to maintaining and improving the existing code, adding this to our definition of done for future additions, and to identify where this is missing to improve that. This work would have to be done at the expense of something else. Removal comes at a low cost in terms of man-hours.

It seem to be en entirely unused feature:

1. We have excluded QDoc from updating the translation files for years now: https://code.qt.io/cgit/qt/qttools.git/tree/src/qdoc/qdoc.pro?h=5.6#n101 - and we've never seen any complaints. 
2. http://l10n-files.qt.io/l10n-files/ contain translation files for Qt, but there’s nothing for QDoc.
3. Our documentation listing of available translations (https://doc.qt.io/qt-5/internationalization.html#locating-qt-translations) does not list QDoc.
4. There are no bug reports coming in from strings that are frequently broken or added.

(minor) This also has a negative impact on both function and line test coverage statistics for QDoc. The proposed solution is to remove this from QDoc for now, and only add it back in if a clear business case can be established.