Remove postcss-filter.js script

Piping CSS files through autoprefixer
is now handled in gulpfile.js

scripts/postcss-filter.js

-#!/usr/bin/env node
-
-// post-process CSS using PostCSS
-// (https://github.com/postcss/postcss)
-//
-// This adds vendor prefixes using autoprefixer
-// https://github.com/postcss/autoprefixer
-
-require('es6-promise').polyfill();
-
-var autoprefixer = require('autoprefixer');
-var postcss = require('postcss');
-
-var inputCss = '';
-
-process.stdin.on('data', function (chunk) {
-  inputCss += chunk;
-});
-
-process.stdin.on('end', function () {
-  postcss([autoprefixer])
-    .process(inputCss)
-    .then(function (result) {
-      console.log(result.css);
-    });
-});
-