math/rkward: remove dependency on WebKit

- it was only a fallback for when WebEngine is not found at buildtime

Qt5Webkit will be removed at the end of the year.

    See: commit e8e65e75

    www/qt5-webkit: mark for deletion

    Qt upstream switched to QtWebEngine quite a while ago, and the maintenance
    of QtWebKit was carried out by a separate person.
    Unfortunately, no release or updates have been pushed in the recent
    years.

    Note: there is a fork [1] of the used fork [2], that might be viable for people that
    really still need to rely on Qt5Webkit.

    [1] https://github.com/movableink/webkit/
    [2] https://github.com/qtwebkit/qtwebkit