qt5+: Drop custom `TextBrowserNoAutoLink` class and use `openLinks`

[fbriere]

The entire purpose of our custom TextBrowserNoAutoLink class was to provide a QTextBrowser where clicking on a link would not automatically open it, which is exactly what the openLinks property is for.

[fbriere]

Note: This was motivated by the removal of the setSource(const QUrl&) QTextBrowser slot in Qt 6 (#297).