Remove unimplemented HierarchicalKeyListJob

* src/Makefile.am: Remove references to hierarchicalkeylistjob.h and
to files generated from it.
* src/hierarchicalkeylistjob.h: Remove.

The class HierarchicalKeyListJob has never been fully implemented. Hence
we can safely remove it without breaking the ABI because nobody can have
used its symbols. The header file doesn't even compile because it
includes cryptobackend.h which never existed since qgpgme was moved to
the gpgme repo.