Remove 'Rendering as Markdown' section

`.livemd` extension was added to github/linguist project in May 2022
(see this PR for details github-linguist/linguist#5672).

Since this time GitHub treats all `.livemd` files as Markdown, and this
section is redundant (or even a little bit misleading).

README.md

@@ -213,24 +213,6 @@ variables used by Elixir releases are also available](
 https://hexdocs.pm/mix/Mix.Tasks.Release.html#module-environment-variables).
 The notables ones are `RELEASE_NODE` and `RELEASE_DISTRIBUTION`.
 
-## Rendering notebooks as Markdown on GitHub
-
-By default GitHub renders the `.livemd` notebooks as regular text files. Depending
-on your use case and the target audience, you may find it useful to render notebooks
-content as Markdown files instead. There is an option to override how a particular
-file gets rendered on GitHub, so all you need to do is add a magic comment in every
-such notebook:
-
-```
-<!-- vim: set syntax=markdown: -->
-
-# My notebook
-
-...
-```
-
-For more details see [the documentation](https://github.com/github/linguist/blob/master/docs/overrides.md#using-emacs-or-vim-modelines).
-
 ## Development
 
 Livebook is primarily a Phoenix web application and can be setup as such: