Remove timestamp_dirty kludge from mark_circuit_unusable_for_new_conns()

In mark_circuit_unsusable_for_new_conns(), we set the unusable_for_new_conns flag... but we also carry around some old code that messes around with timestamp_dirty.

The root problem here is that the old code makes timestamp_dirty into a lie; "unusable" and "dirty" are not the same concept.

We should carefully audit timestamp_dirty and its users to make sure that it's safe to remove this old kludge, and then remove it or replace it with something more accurate.