Remove Doxygen experiment from the code

For a couple of years, we added some support for doxygen and some documentation is the code. At one given moment we even had code.musescore.org pointing to a doxygen generated documentation of the code... Nobody cared much and so we killed it.

Werner, the lead developer of MuseScore, doesn't like big headers before functions, also according to him there is no value in documentation without the code. It could make sense if MuseScore was a library, but currently it's not the case. Werner would prefer to have meaningful function names and variables names and in place where it's needed to have inline comments to document the algorithms. Obviously some place in the code (beam.cpp I'm looking at you) should get this treatment.

In the meantime, let's get rid of Doxygen.