dirstate: remove need_delay logic
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.
[1] except the ones from `hg update`, but `need_delay` no longer help for them
either.
Differential Revision: https://phab.mercurial-scm.org/D11796
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