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/input/README.org

@@ -1,4 +1,3 @@
-# -*- mode: doom-docs-org -*-
 #+title:   :input
 #+created: May 13, 2019
 #+since:   2.0.9

modules/input/bidi/README.org

@@ -1,4 +1,3 @@
-# -*- mode: doom-docs-org -*-
 #+title:    :input bidi
 #+subtitle: (tfel ot) thgir etirw uoy gnipleh
 #+created:  April 22, 2022

modules/input/chinese/README.org

@@ -1,4 +1,3 @@
-# -*- mode: doom-docs-org -*-
 #+title:    :input chinese
 #+subtitle: Spend your 3 hours a week in Emacs
 #+created:  May 13, 2019

modules/input/japanese/README.org

@@ -1,4 +1,3 @@
-# -*- mode: doom-docs-org -*-
 #+title:    :input japanese
 #+subtitle: Ah, a man of culture
 #+created:  May 13, 2019

modules/input/layout/README.org

@@ -1,4 +1,3 @@
-# -*- mode: doom-docs-org -*-
 #+title:    :input layout
 #+subtitle: auie,ctsrnm is the superior home row
 #+created:  Jun 29, 2020