remove population subqueries

[missinglink]

DRAFT, for now, this will require updates to the whosonfirst importer (or pelias/model?) before merging in order to ensure no breaking changes.

This PR removes the population subqueries, as per the discussion in https://github.com/pelias/openstreetmap/pull/493#issuecomment-503025012 we could copy population info to the popularity field for admin areas and have one less subquery.

There is some momentum building at the moment to improve popularity scoring (which was always enabled but rarely used). If we decide to go down that path then this PR will remove all the population subqueries.

There should be a PR in pelias/whosonfirst or pelias/model before this is merged which is responsible for setting the popularity.

Note: if the schema and API are out-of-sync then admin areas could receive a 'double-boost' until this code is deployed, if that is a concern then it might be better to 'move' population info to popularity (setting population to 0) for a short migration period. Worth checking if there are differences in boost values between the fields in any of the defaults.

Finally, if we truly never need to search on population then pelias/schema could be updated to ensure that the field isn't indexed.