Remove node-pngquant-native dependency

- Unnecessary native dependency
- It was disabled by default
- Has issues on certain platforms
- Not optimal for production use (performance)

package.json

@@ -34,7 +34,6 @@
     "handlebars": "4.0.10",
     "http-shutdown": "^1.2.0",
     "morgan": "1.8.2",
-    "node-pngquant-native": "1.0.5",
     "nomnom": "1.8.1",
     "pbf": "3.0.5",
     "proj4": "2.4.4",