babel: Drop CommonJS plugin

Now all `module.exports` are gone, so this can be dropped again. This
tightens the static (compile time) import/export checks and avoids
runtime errors due to typos, undeclared exports, etc. This also enforces
consistent import/export handling from now on.

Closes #10890

webpack.config.js

@@ -379,8 +379,7 @@ var babel_loader = {
                 }
             }],
             "@babel/preset-react"
-        ],
-        plugins: ["@babel/plugin-transform-modules-commonjs"]
+        ]
     }
 }