Adapt chosen (css) path

Avoid shipping duplicate files in 3rdparty/css.

3rdparty/css/chosen/ already contains chosen.css and chosen-sprite.png
so its useless to duplicate them in 3rdparty/css.

Taking the same approach as in apps/calendar/appinfo/app.php and
apps/files_external/settings.php.
Merge pull request #8309 from DavidPrevot/chosen

Adapt chosen (css) path