Documentation/magic-number: Remove SCI_MAGIC

The SCI_MAGIC define was removed in commit f8e53553 ("serial:
sh-sci: Kill off more unused defines.").
Its last user was the old drivers/char sh-sci driver, which was removed
in 2004.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>