iris: Delete bogus comment about cube array counting.

iris: Delete bogus comment about cube array counting.

Both 'z' and 'depth' are counted in slices, according to the Gallium
docs (context.rst).  In our temporary memory, we allocate `box.depth`
slices, so we need to rebase the starting slice (box.z) down to 0,
and back again when writing on unmap.

There's nothing strange about cubes here.