remove deprecated and unused man_made=power_wind

Used 0 times worldwide - http://taginfo.openstreetmap.org/tags/man_made=power_wind#overview
Deprecated according to wiki - http://wiki.openstreetmap.org/wiki/Tag:man_made%3Dpower_wind

I confirmed that this change is not introducing syntax errors, and test data generated the same images as https://github.com/mkoniecz/hg