Mark these pkgdb examples as Legacy

This needs scrutiny from @pypingou and @tyll.

By marking these tests as ``Legacy``, they will no longer show up in the
docs at http://fedora-fedmsg.readthedocs.org/en/latest/topics.html

In fedora-infra/fedmsg#255, @tyll mentioned that some pkgdb1 messages
still appeared in the docs even though they are not being broadcast
anymore.  I did my best to check that the topics of these messages that
I've marked do not actually appear in the pkgdb2 source, but I'd be much
more comfortable merging this if someone more familiar with the source
could look over them.

If merged, this fixes fedora-infra/fedmsg#255.