Now that all¹ stored mtime are non ambiguous, we no longer need to apply the need_delay step.
The need delay logic was not great are mtime gathered during longer operation
could be ambiguous but younger than the dirstate.write call time.
So, we don't need that logic anymore and can drop it
This make the code much simpler. The code related to the test extension faking
the dirstate write is now obsolete and associated test will be migrated as
follow up. They currently do not break.
 except the ones from hg update, but need_delay no longer help for them