I have never understood why this class was added in the first place,
and digging through the old commit history does not help much in this
regard.  It was mostly used as another way of doing signals and slots.

 * The calls in ArchiveInterface that called each observer method now
   just emit the signals declared in ReadOnlyArchiveInterface, and the
   observer methods in the Job classes are now slots.
 * ReadOnlyArchiveInterface::removeObserver() has been replaced with
   QObject::disconnect().

All the (few) unit tests we have still pass.