Remove the broken 'Marker.size' property
Remove the broken 'Marker.size' property

This merges branch 'remove-marker-size' which removes the property.

The property never worked -- it would always have emitted a `KeyError`.
Remove it so that we don't need to document it or make it work. If we
add it later, we can add docs etc. then.

sourcebots/robot-api,sourcebots/robot-api