iscsi: simplify iscsi_co_discard

now that bdrv_co_discard can handle limits we do not need
the request split logic here anymore.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>