Remove recursion from kill_block()

Remove recursion from kill_block()

The recursion can be converted to a conditional. Why is it not
converted to a while-loop? It could be, but there should be no
difference in practice, because mergeable blocks are never created
(instead the previous/following block is extended).

Note that looping over following blocks was never possible as long
as the function guaranteed that it doesn't destroy anything that
follows the block to be deleted.