Remove old/* plugins.

It's been almost 3 years since they were moved to the old/ folder, and
apparently they've not been needed enough by anyone to get an update
to current plugin API. If ever they're deemed useful again, they can
be pulled back via Git and updated.
Remove `open_in_gui.py` plugin.

The functionality is included in Picard's core now, so there is no
need for a plugin to do it as well.
Merge pull request #255 from Freso/cleanup-contrib-plugins

Clean up contrib plugins