Radar Dataset property "pointHighlightStroke" used in samples does not exist

[moberwasserlechner]

The property is used in both radar samples but neither exists in the documentation or code.

It should be removed from samples/radar-skip-points.html and samples/radar.html.

Search query: https://github.com/chartjs/Chart.js/search?utf8=%E2%9C%93&q=pointHighlightStroke

[etimberg]

Fixed in #3027