dirstate: directly call the dirstatemap in `set_untracked`
This function is only called in two places: the deprecated "remove" method and
in the new `set_untracked` method.
So we simply inline the appropriate content in `set_untracked`, paving the way
to dropping the deprecated code and its associated function.
Differential Revision: https://phab.mercurial-scm.org/D11313
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