Remove Qt Assistant executable from build

We should remove the Qt Assistant executable from Qt 6:

• We should concentrate on online documentation and documentation in Qt Creator
• The default QTextBrowser backend prevents generation of HTML5 compliant html
• Qt Assistant functionality is independent of the bit version / compiler used, so building and shipping it for every single build variant separately is a waste of resources
• Qt Assistant is also not bound to the Qt major version. Taking it out of Qt 6 will mean distributions etc can use always the latest version, even for Qt 5 documentation.