dagutil: remove descendantset() and ancestorset()
descendantset() is unused after the previous commit. And
ancestorset() was only used by descendantset(), so it can be removed
as well.
.. api:: descendantset() and ancestorset() removed from dagutil
Use a revset instead when operating on the changelog. Or use
various functionality in the ancestor or dagop modules.
Differential Revision: https://phab.mercurial-scm.org/D4315