kernel-doc: drop looking for "MACDOC"

Linux kernel does not use "MACDOC" in any documenation or any
source files, so stop searching for it.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240108003700.13418-1-rdunlap@infradead.org