grab one minimizer rather than three

Apparently WebPack builds in "terser", which is what the cool kids use.
Just go with the default and simplify the configuration as well as
installing fewer node modules.

"babel-minify-webpack-plugin" wasn't actually being used, and
babel-minify is still in beta anyway.

uglifyjs, according to https://github.com/webpack/webpack/issues/7923,
doesn't support ES6 and depends on a package which is no longer
maintained.

webpack/prod.config.js

@@ -54,33 +54,6 @@ module.exports = (env, args) => {
     },
     optimization: {
       minimize: true,
-      minimizer: [
-        {
-          apply: (compiler) => {
-            /**
-             * Setup the UglifyJsPlugin as webpack4 does, plus options
-             * we decide to override.
-             */
-            // Lazy load the uglifyjs plugin
-            const UglifyJsPlugin = require('uglifyjs-webpack-plugin');
-            new UglifyJsPlugin({
-              cache: true, // webpack4: default
-              parallel: true, // webpack4: default
-              sourceMap: true,
-              uglifyOptions: {
-                compress: {
-                  keep_infinity: true, // Do not change to 1/0
-                  warnings: false, // Do not warn when dropping
-                },
-                output: {
-                  // Eliminate most comments, but not marked ones
-                  comments: 'some',
-                },
-              },
-            }).apply(compiler);
-          },
-        },
-      ],
       splitChunks: {
         minSize: 30000,
         minChunks: 1,