turnip: Fix blitting with cubic filtering

When the current blit library was first written we just blindly copy/pasted some bits from the blob's sampler descriptor, but with cubic filtering we hit a case where one of the bits is actively harmful. Document it for posterity/the curious and remove it.

This fixes most of the blit-from-compressed-texture tests added to vulkan CTS 1.2.8, but it seems BC6h is still broken somehow.