Remove CStdGL::BlitLandscape

This code is only used for the low-resolution landscape that is hardly in use
anymore. The code was mostly a duplicate of the standard C4Surface blit
function, CStdGL::PerformBlt, with some added code for blitting material
textures with higher resolution. However, that code was not enabled anymore
by the classic landscape renderer either, so it seems safe to remove it.

The landscape is now simply drawn by C4Draw::Blit.
Remove unused code
Remove config entries that are no longer used