Doc: Remove use of defunct QDoc command "\tableofcontents"

Across Qt's source, the defunct QDoc command `\tableofcontents` is sometimes preceded by leading text, e.g. "Topics:", which makes it appear as if something is missing from the output. See for example how https://doc.qt.io/qt-6/qstyle.html#details ends with "Topics:", indicating that it should be followed by the ToC as indicated by the use of the command:
(https://github.com/qt/qtbase/blob/dev/src/widgets/styles/qstyle.cpp#L133). However, this ToC isn't generated by QDoc as the command has been made redundant.

Remove the ~80 uses of \tableofcontents across modules, and ensure that surrounding context still makes sense for the reader.