Drop dependency on www/qt5-webkit.

Reduce qt5-assistant's dependency tree by dropping the dependency on QtWebKit.
Instead, use qt5-widgets' QTextBrowser to render documentation. An added bonus
is that with this change www/qt5-webengine no longer depends on www/qt5-webkit.

Page rendering is slightly worse, but everything is still very much readable.

Reviewed by: tcberner
Differential Revision: https://reviews.freebsd.org/D13630