Remove files of different buildsystems that are either useless or
unusable, so to not clutter the sources and confuse the users.

Drop the qmake .pro file: it claimed to be there for Qt Creator;
however Creator supports cmake natively these days, so it is no more
needed. Furthermore, this .pro file is not complete, e.g. does not
generate the needed version.h.

Drop the no-op 'configure' script: cmake is widely known nowadays, so
people know how to build using cmake without the need of a reminder.