Editor: Remove back-compat stub for Classic Editor's old distraction-free writing mode.

The original script was deprecated in [30338] and removed in [32677].

Props Presskopp, ketuchetan.
Fixes #39448.
Built from https://develop.svn.wordpress.org/trunk@47164


git-svn-id: https://core.svn.wordpress.org/trunk@46964 1a063a9b-81f0-0310-95a4-ce76da25c4cd