Remove MAPNIK_GIT from appveyor.yml
Remove MAPNIK_GIT from appveyor.yml

MAPNIK_GIT is now pulled from `package.json`.
mapnik/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,CartoDB/node-mapnik,CartoDB/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,tomhughes/node-mapnik,CartoDB/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,tomhughes/node-mapnik,mapnik/node-mapnik,CartoDB/node-mapnik,CartoDB/node-mapnik