docs(guides): revert changes from #1158 (#1362)

This tip is no longer necessary as webpack/webpack#4771 was
fixed.

Author: richardsimko

content/guides/author-libraries.md

@@ -129,8 +129,6 @@ module.exports = {
 
 This means that your library expects a dependency named `lodash` to be available in the consumer's environment.
 
-If your library targets UMD, it's important to add all of the above mentioned ways of loading the external (`commonjs`, `commonjs2`, `amd` and `root`) as leaving one out will cause strange errors for a consumer trying to load your library in that environment.
-
 If you only plan on using your library as a dependency in another webpack bundle, you may specify externals as an array.
 
 ```javascript