Investigate use of both overlay and _draw_overlay in AbstractOverlays

For the full saga, see this comment thread: https://github.com/enthought/chaco/pull/743#discussion_r634611098

The confusion comes from chaco defining its own `AbstractOverlay` which ends up calling the normal `_draw` method if its `component` is `None`. ie, chaco `AbstractOverlays` can be drawn standalone like any other component.
See:
https://github.com/enthought/chaco/blob/5d692b382770c6ac97d8e4673875a54cc072ee59/chaco/abstract_overlay.py#L52-L62

Conversely in enable:
https://github.com/enthought/enable/blob/6e1c93e9df2a6e37eb79b92a81696ac757392dce/enable/abstract_overlay.py#L84-L90
that is not the case, so it would not be necessary to have a `_draw_component` method on the class (since there is no code path where it would be called)

The real question is, do all overlays in chaco need the ability to be drawn standalone? Some definitely do need to keep the `_draw_overlay` method as they do their own special things, eg `Tooltip` and its subclass `DataLabel`. However, it is unclear to me if `PlotAxis` or `Grid` actually need it. AFAICT, `PlotAxis` will always have its `component` set when used inside the chaco code base. I _think_ its `_draw_overlay` method could be removed.

Note, this is overall probably low priority / the removals would only be to simplify code / make it more obvious what methods are for. Even if a method is unused, it isn't causing to much trouble. It may just be some added confusion for developers trying to distinguish which method does what / why both exist, etc.
More thorough documentation may be the simpler / less risky solution here.