Remove polygon-gamma

polygon-gamma was left over from the XML conversion, and doesn't actually
seem to help anything

Fixes #73
Superceeds #867
Clean up and reformat water area SQL

This reformats the water SQL, and eliminates unused and hardly used
mistaken taggings for water.

Cross-reference #204
Don't return way_area for water areas

We're not using way_area, so don't return it
Filter out small areas and use polygon-gamma for large ones

This should fill in gaps between water bodies, prevent colouring bugs
and not over-represent water at low zooms
Turn down polygon-gamma even more for bigger areas

This completely eliminates white lines where large areas join.
Merge pull request #878 from pnorman/polygon-gamma

Be selective when applying polygon-gamma to water areas, and remove a few old tags.