Remove node-pngquant-native dependency

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

docs/config.rst

@@ -21,9 +21,7 @@ Example::
       ],
       "formatQuality": {
         "jpeg": 80,
-        "webp": 90,
-        "pngQuantization": false,
-        "png": 90
+        "webp": 90
       },
       "maxScaleFactor": 3,
       "maxSize": 2048,
@@ -83,8 +81,6 @@ Use ``false`` to disable the front page altogether (404).
 
 Quality of the compression of individual image formats. [0-100]
 
-The value for ``png`` is only used when ``pngQuantization`` is ``true``.
-
 ``maxScaleFactor``
 -----------