Label placement layers configuration should come from style

[zerebubuth]

At the moment, tilequeue reads a label-placement-layers configuration from the RAWR section of its own configuration. However, this really ought to be a style option in queries.yaml, as it needs to be coordinated with the SQL queries.

For context, see https://github.com/tilezen/tileops/pull/42 and https://github.com/tilezen/vector-datasource/issues/1702.