Remove work limit in UpdateForDescendants()

The work limit served to prevent the descendant walking algorithm from doing
too much work by marking the parent transaction as dirty. However to implement
ancestor tracking, it's not possible to similarly mark those descendant
transactions as dirty without having to calculate them to begin with.

This commit removes the work limit altogether. With appropriate
chain limits (-limitdescendantcount) the concern about doing too much
work inside this function should be mitigated.