docs/mm: Physical memory: Remove zone_t

"zone_t" doesn't exist in current code base anymore, remove the
description of it.

Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250115070355.41769-1-richard120310@gmail.com