Remove local government document filter

The filter hasn't been used on production for a long while, but we were
still keeping the search index up to date for it. This meant reindexing
every edition related to a policy when that policy was reindexed.

This commit pulls out the filter code and also the indexing code that
was used to enable it.
Reindex collection documents when collection indexed
Merge pull request #1129 from alphagov/reindex-collection-documents-when-collection-changes

https://www.pivotaltracker.com/story/show/58228020

Reindex collection documents when collection changes