Don't use babel when running tests using karma (#398)

Currently instrumentation is running after babel, which results in
incorrect and nonsensical coverage information. The node vs. web
coverage here is an example:
https://codecov.io/gh/GoogleChromeLabs/confluence/src/f6802df5a6be89042a1db1e7b82f0a0a90906caf/lib/dao/indexed_dao.es6.js

This could be fixed by changing the order:
#404

However, it's not clear if that's correct:
webpack-contrib/istanbul-instrumenter-loader#86

There's no need to run Babel to test stable versions of Chrome or Firefox,
so just skip babel-loader.

config/karma.conf.js

@@ -107,15 +107,6 @@ configurator.webpackConfig = {
   mode: 'development',
   module: {
     rules: [
-      {
-        test: C.ES6_REG_EXP,
-        use: [
-          {
-            loader: 'babel-loader',
-            options: C.ES6_LOADER_OPTIONS_DEV,
-          },
-        ],
-      },
       {
         test: /\.html$/,
         use: [{loader: 'html-loader'}],