Documentation/magic-number: Remove SCC_MAGIC

The SCC_MAGIC define was removed in commit 1ec3ba93 ("Staging:
remove drivers/char/scc.h as it should have gone a while ago"), while
its actual last user was removed before in commit bb2a97e9
("Staging: delete generic_serial drivers").

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