Remove window hints clearing the flag Qt::WindowContextHelpButtonHint,
which is already off by default in Qt 6.
In Qt 5 this flag was set by default for QDialogs, and on Windows put a
? button in the corner of the title bar allowing users to activate Qt's
QWhatsThis help system for a given widget. Since we don't set that text
the ? button was useless and so we hid it manually.
