Remove section about ES modules and tree-shaking (#3410)

In practice, tree-shaking does not currently allow for the kind of
bundle size reduction achievable using direct module imports, so we
shouldn't encourage it as an option for minimizing bundle size.

docs/guides/MinimizingBundleSize.md

@@ -1,21 +1,16 @@
 # Minimizing Bundle Size
 
-For convenience, React Router exposes its full API on the top-level `react-router` import. However, this causes the entire React Router library and its dependencies to be included in client bundles that include code that makes any imports from the top-level CommonJS bundle.
+For convenience, React Router exposes its full API on the top-level `react-router` import. However, this causes the entire React Router library and its dependencies to be included in client bundles that include code that imports from the top-level CommonJS bundle.
 
-There are two options for minimizing client bundle size by excluding unused modules.
+Instead, the bindings exported from `react-router` are also available in `react-router/lib`. When using CommonJS models, you can import directly from `react-router/lib` to avoid pulling in unused modules.
 
-
-## Import from `react-router/lib`
-
-Bindings exported from `react-router` are also available in `react-router/lib`. When using CommonJS models, you can import directly from `react-router/lib` to avoid pulling in unused modules.
-
-Assuming you are transpiling ES2015 modules into CommonJS modules, instead of
+Assuming you are transpiling ES2015 modules into CommonJS modules, instead of:
 
 ```js
 import { Link, Route, Router } from 'react-router'
 ```
 
-use
+use:
 
 ```js
 import Link from 'react-router/lib/Link'
@@ -24,16 +19,3 @@ import Router from 'react-router/lib/Router'
 ```
 
 The public API available in this manner is defined as the set of imports available from the top-level `react-router` module. Anything not available through the top-level `react-router` module is a private API, and is subject to change without notice.
-
-
-## Use a Bundler with ES2015 Module Support
-
-React Router offers a ES2015 module build and defines a `jsnext:main` entry point. Only if you are using a bundler that supports ES2015 modules and tree-shaking such as webpack 2 or Rollup, you can directly import from `react-router`, as long as you are correctly resolving to the ES2015 module build. Specifically, in those cases, you can write
-
-```js
-import { Link, Route, Router } from 'react-router'
-```
-
-Tree-shaking will ensure that only the relevant modules will be included in your bundle.
-
-**Note:** Please do not import from the subdirectory with the ES module build directly. If you are using webpack 2, please add `jsnext:main` to `resolve.mainFields` (or `resolve.packageMains` for older versions of webpack 2).