This patch replaces the old Qt logo with the new, flatter one.

The PNGs were optimized via: optipng -o7 -strip "all"

src/assistant/assistant/images/qt-logo.png was deleted since it
is unused.

Task-number: QTBUG-41685
Change-Id: Id04972f87135e15c41d52c5e9f00ad4944d8d1f6
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>