Streamline Google Traffic tiles

[goldfndr]

This change removes the text and POI icons. Although "transit" is part of the request, it doesn't have any visible impact (from my spot-checks). The reduced "avg_img_size" is simply a guess; I don't know what method is used to calculate it, but it's definitely much smaller than before.

I stayed with the existing style=15, but there are numerous other style values that might be preferable. I wasn't able to get tiles to appear at zoom levels 4 or below via OsmAnd so I changed min_zoom.