revlog: no longer return the nodemap after parsing
Now that both pure and cext version have a `nodemap` attribute, we do not need
to return the `nodemap` object after parsing.
This work is part of a refactoring to unify the revlog index and the nodemap.
This unification prepare the use of a persistent nodemap.
Differential Revision: https://phab.mercurial-scm.org/D7314
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