Remove commented out line from platforms.pro

No reason to add this. It's commented out and minimal is added
further down.

Task-number: QTBUG-29534
Change-Id: If63cd0e1ac266acf583f27225adb9ba7a75016ea
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Reviewed-by: BogDan Vatra <bogdan@kde.org>