Remove node-pngquant-native dependency

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

src/serve_rendered.js

@@ -18,7 +18,6 @@ var Canvas = require('canvas'),
     mercator = new (require('@mapbox/sphericalmercator'))(),
     mbgl = require('@mapbox/mapbox-gl-native'),
     mbtiles = require('@mapbox/mbtiles'),
-    pngquant = require('node-pngquant-native'),
     proj4 = require('proj4'),
     request = require('request');
 
@@ -451,16 +450,6 @@ module.exports = function(options, repo, params, id, dataResolver) {
             return res.status(404).send('Not found');
           }
 
-          if (format == 'png') {
-            var usePngQuant =
-                (options.formatQuality || {}).pngQuantization === true;
-            if (usePngQuant) {
-              buffer = pngquant.compress(buffer, {
-                quality: [0, formatQuality || 90]
-              });
-            }
-          }
-
           res.set({
             'Last-Modified': lastModified,
             'Content-Type': 'image/' + format