Drop lanczos filter

The lanczos filter depends on the effect system. It makes very difficult
changing painting code from SceneWindow to Item.

Given that the last big users of the laczos filter - present windows and
desktop grid effects were re-written in QML. The only remaining user of
the lanczos filter is the thumbnail aside effect. Given that it's a
really obscure usecase, switching to the linear filter won't be very
noticeable.

As a backup plan, one can reimplement the thumbnailaside effect using
QML. The lanczos filter is already implemented in plasma-framework.