Remove support for generating DITA XML output from QDoc

Qdoc's DITA XML generator has slipped further and further out of date, so that now it no longer generates useful output. The output was based on a specialization of the DITA DTDs for C+, but this specialization was more useful for generating DITA to represent the C+ code than to document it, and it didn't include support for QML, which Qt5 needs. It is expected that if QDoc users decide they need DITA XML support, a new DITA specialization will be created that better represents what QDoc needs.