Remove panopticon registration task.

This was only for campaigns, and these should not be registered, as
they're not wanted in search.

This task was not being run on deploy (or at any other time), but its
presence provides confusion, and could lead to mistakes.
Remove registerable_campaign as it's no longer needed
Merge pull request #366 from alphagov/remove_panopticon_registration

Remove panopticon registration task.