Revert "wined3d: Preload target in ActivateContext() for ORM_BACKBUFFER/ORM_PBUFFER."

[Stefan Dösinger]

> Two Preload calls in FindContext. And bug 13079:
> http://bugs.winehq.org/show_bug.cgi?id=13079
Ah, I think 13079 is an entirely different bug. Vertex buffers aren't
involved in offscreen rendering readback, nor in render target selection.
(Currently. D3D10 changes that)