Remove @mdx-js/register

Author: wooorm

docs/docs/getting-started.mdx

@@ -913,10 +913,8 @@ more info.
 #### Node.js
 
 MDX files can be imported in Node by using
-[`@mdx-js/node-loader`][mdx-node-loader] (*strongly recommended*) or
-alternatively they can be `require`d with the legacy package
-[`@mdx-js/register`][mdx-register].
-See their readmes on how to configure them.
+[`@mdx-js/node-loader`][mdx-node-loader].
+See its readme on how to configure it.
 
 ## Further reading
 
@@ -952,8 +950,6 @@ See their readmes on how to configure them.
 
 [mdx-react]: /packages/react/
 
-[mdx-register]: /packages/register/
-
 [mdx-rollup]: /packages/rollup/
 
 [mdx-vue]: /packages/vue/