manifest: remove unused property _oldmanifest
The last use seems to have gone away in
7c7d845f8b64 (manifest: make
manifestlog use it's own cache, 2016-11-10).
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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273ce12ad8f1` will report some style violations on a very old commit).