rhg: fix dirstate-v2 data file removal system
In D12581 I introduced logic to remove the previous dirstate-v2 data file
after a new one is created (and its corresponding docket), but the logic was
flawed. I fixed it and made it simpler to understand by gather all logic in
a single expression.
Differential Revision: https://phab.mercurial-scm.org/D12586
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