dirstate: drop duplicated check
This is covered by the `@requires_parents_change` decorator that this function use.
Differential Revision: https://phab.mercurial-scm.org/D11125
dirstatemap: use `set_possibly_dirty` in `clearambiguoustimes`
lets put this new method to use
Differential Revision: https://phab.mercurial-scm.org/D11121