removing waterway=canal area rendering

[imagico]

fixes #2424.

Note this only removes the area rendering, correctly treating closed ways with waterway=canal as line features will depend on the database reload. Current lua branch already has this covered:

https://github.com/gravitystorm/openstreetmap-carto/blob/lua/openstreetmap-carto.lua#L40

[pnorman]

Current lua branch already has this covered:

https://github.com/gravitystorm/openstreetmap-carto/blob/lua/openstreetmap-carto.lua#L40

Not really. There's two different questions - when waterway=canal is an area, and rendering waterway=canal areas. Lua changes the first, this PR changes the second.

[imagico]

This is exactly what i meant - right now this PR removes the incorrect rendering of closed way canals as areas and this will make sense also after the database changes but it does not make sure they are rendered correctly as line features since the geometries are still treated as polygons in the database.

[pnorman]

Looks good to me, but I haven't had a DB loaded to actually test.