revert: fix(docs): set mode in file-local vars

emacs-straight/org-mode@e22b4eb7aa introduced a breaking change that
made it impossible to rely on .dir-locals.el to change org-mode buffers
in $EMACSDIR to doom-docs-org-mode (without infinite recursion), so we
used file-local variables in 7e400ab.

emacs-straight/org-mode@215de6176b reverted that change, so we can use
.dir-locals.el again, and this is my preference, since it requires less
boilerplate across our hundreds of org files.

Ref: emacs-straight/org-mode@215de6176b
Ref: emacs-straight/org-mode@e22b4eb7aa
Revert: 7e400abdc0

modules/os/README.org

@@ -1,4 +1,3 @@
-# -*- mode: doom-docs-org -*-
 #+title:   :os
 #+created: November 03, 2021
 #+since:   21.12.0

modules/os/macos/README.org

@@ -1,4 +1,3 @@
-# -*- mode: doom-docs-org -*-
 #+title:    :os macos
 #+subtitle: Compatibility for our favorite walled garden
 #+created:  February 19, 2017

modules/os/tty/README.org

@@ -1,4 +1,3 @@
-# -*- mode: doom-docs-org -*-
 #+title:    :os tty
 #+subtitle: Make TTY Emacs suck less
 #+created:  August 07, 2020