traffic signals labels should be moved from junction layer to amenity-points

`highway=traffic_signals` labels are currently rendered in the junctions layer (minzoom 11) and are displayed from z15 and then with an offset to make space for the symbol, which is separately rendered in `amenity-points`, starting z17. This is not good for several reasons: * inefficiency since the junction layer query will include all traffic signals from z11. * the label has higher priority than all POI symbols. * named traffic signals are rare, <4 percent, and most of these are probably named crossings/junctions tagged with `highway=traffic_signals` instead of `highway=crossing` as tagging for the renderer (5 percent of all `highway=traffic_signals` also have `crossing=traffic_signals`). I would suggest to either move the traffic signals labels to amenity-points or to remove them completely.