Rummager doesn't support format filter

Removed from Rummager here:
https://github.com/alphagov/rummager/commits/9eea3969004b31588d27d037f8df34ac99748b95
Remove obsolete Rummager autocomplete code

It was removed from Rummager here:
https://github.com/alphagov/rummager/commits/f83c49fd9b75ba201ddb1b1ba10221c82db65e57
Merge pull request #64 from alphagov/remove_obsolete_search_features

Remove obsolete search features