dagutil: remove heads() and localsubset from revlogdag.__init__
The previous commit removed the last consumer of this API.
I'm not going to mark as API incompatible because I doubt anybody
used this functionality (outside of possibly passing an argument
to revlogdag.__init__). I intend to remove revlogdag later in
this series and its API annotation will cover this one.
Differential Revision: https://phab.mercurial-scm.org/D4320
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