Remove the unused ALLOWED_RETURN_FIELDS constant
Remove the unused ALLOWED_RETURN_FIELDS constant

It hasn't been used since f862c694c5a77d227b91692dbcc22f07d478327c,
which changed to use the schema to generate the list of allowed return
fields.

theodi/rummager,theodi/rummager,alphagov/rummager,alphagov/rummager