Removed groupByKey function, accidentally leaked from another branch.
Removed groupByKey function, accidentally leaked from another branch.

ebrelsford/leaflet-tilelayer-vector,kozer/leaflet-tilelayer-geojson,Livefyre/leaflet-tilelayer-geojson,glenrobertson/leaflet-tilelayer-geojson,jgimbel/leaflet-tilelayer-geojson,ebrelsford/leaflet-tilelayer-vector,nrenner/leaflet-tilelayer-vector