Disable the uglify plugin

We have an old version of uglify, and it's causing problems with
compiling d3-array, and also when upgrading to a newer babel.

I'm going to disable it for now, then upgrade babel, and then upgrade
webpack. Upgrading webpack will get us a later version of uglify without
these issues.

config/webpack.config.web.js

@@ -254,27 +254,6 @@ function plugins(mode /*: "development" | "production" */) {
   const prodOnlyPlugins = [
     // Remove the output directory before starting the build.
     new RemoveBuildDirectoryPlugin(),
-    // Minify the code.
-    new webpack.optimize.UglifyJsPlugin({
-      compress: {
-        warnings: false,
-        // Disabled because of an issue with Uglify breaking seemingly valid code:
-        // https://github.com/facebookincubator/create-react-app/issues/2376
-        // Pending further investigation:
-        // https://github.com/mishoo/UglifyJS2/issues/2011
-        comparisons: false,
-      },
-      mangle: {
-        safari10: true,
-      },
-      output: {
-        comments: false,
-        // Turned on because emoji and regex is not minified properly using default
-        // https://github.com/facebookincubator/create-react-app/issues/2488
-        ascii_only: true,
-      },
-      sourceMap: shouldUseSourceMap,
-    }),
   ];
   switch (mode) {
     case "development":