pens: deprecate 'ignore_single_points' argument
pens: deprecate 'ignore_single_points' argument

This was needed for when we were supporting the old robofab.ufoLib
where anchors were represented as single-point-with-a-name contours.
With latest ufoLib, even when glif format is 1, anchors are
stored separately and are not drawn with pens (only proper contours
and components are passed on in the draw method).
So there is no longer need to special-case single points.
It's not the job of the cu2qu pens to filter those out.

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