Unset the document_id field on PanopticonMappings

We switched from document_id to resource_id and resource_type, so we no
longer use this information, but ~105 PanopticonMappings still have a
document_id attribute.

Nuke it from orbit.
Merge pull request #149 from alphagov/remove_document_id_legacy_field

Unset the document_id field on PanopticonMappings