doc: update style guide for markdown extension

There are no longer files in the repository that use the `.markdown`
extension so remove mention of them.

PR-URL: https://github.com/nodejs/node/pull/15786
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Lance Ball <lball@redhat.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

doc/STYLE_GUIDE.md

@@ -6,8 +6,6 @@
     topic the document will describe (e.g., `child_process`).
   * Filenames should be **lowercase**.
   * Some files, such as top-level markdown files, are exceptions.
-  * Older files may use the `.markdown` extension. These may be ported to `.md`
-    at will. **Prefer `.md` for all new documents.**
 * Documents should be word-wrapped at 80 characters.
 * The formatting described in `.editorconfig` is preferred.
   * A [plugin][] is available for some editors to automatically apply these