Remove way_area from water_polygons

[pnorman]

ref #pnorman/spirit#52

The water_polygons layer requires a way_area attribute be added to all objects. A map style might need way_area for the label point, but not for the polygon. In practice the polygon is always used for a fill and ordering doesn't matter.

Additionally, this prevents any useful merging of water areas to reduce object count.

cc @someoneelseosm

[pnorman]

Removing this and merging reduced the size of a z7 tile by roughly 15% so the gains are significant

[SomeoneElseOSM]

FWIW https://github.com/SomeoneElseOSM/SomeoneElse-vector-web-display/blob/main/resources/svwd03_style.json#L6491 (which works with OSMF's demo Shortbread tiles) uses way_area on water_polygons_labels only. I can't really see a need for way_area on the polygons themselves.

In a different schema / style I make extensive use of way_area when deciding what to include in a particular tile when creating tiles, but only for centroid display in the style itself.

[pnorman]

I would be open to dropping this in 1.1, but I've assigned it to the 2.0 milestone.