gallium/util: remove some block alignment assertions

gallium/util: remove some block alignment assertions

These assertions were revisited a couple of times in the past, and they
still weren't quite right.
The problem I was seeing (with some other state tracker) was a copy between
two 512x512 s3tc textures, but from mip level 0 to mip level 8. Therefore,
the destination has only size 2x2 (not a full block), so the box width/height
was only 2, causing the assertion to trigger for src alignment.
As far as I can tell, such a copy is completely legal, and because a correct
assertion would get ridiculously complicated just get rid of it for good.

Reviewed-by: Brian Paul <brianp@vmware.com>