(1.21.1) Fix bug with mixed slabs and waterlogging

[throckmorpheus]

When you placed a different slab type onto a waterlogged slab to create a double slab, then broke the slab that hadn't been waterlogged, the remaining slab would return to a waterlogged state (but not flow out until updated).

This fixes it so the water is removed, like what happens with two slabs of the same type (and vertical slabs, which never had this issue).