Clean up duplicate search index entries.

A whole bunch of statistics publications have duplicate entries with /statistics and /publications in the paths.
eg. https://www.gov.uk/government/statistics?departments%5B%5D=environment-agency
There are no duplicate publications on the admin side, this is limited to the search index.
They appear to be specific to the Environment Agency and to particular
subjects which might imply a workflow problem. They need de-duping
in any case.
https://www.pivotaltracker.com/story/show/76045766