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.

Fix #8048

reduce code duplication, fix parse error, prevent page reload on hitting enter while changing the display name - refs #8085

This file seems not to be actually used anywhere.