Remove useless code from qdoc

The documentation used to have "Main Classes" list for C++ classes, and qdoc kept track which classes were supposed to be listed as "main" classes. This is no longer used, so this change removes the code that marked a C++ class as a main class and tested whether a class was a main class. This dead code was seen while preparing the way for changing qdoc to output a documentation page for obsolete C++ classes and functions and obsolete QML types and functions