docs/memory-barriers.txt: Remove remaining references to mmiowb()

This commit removes references to sections erased by Commit 91553039
("Documentation: Kill all references to mmiowb()").

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Link: https://lore.kernel.org/r/20191121234125.28032-6-sj38.park@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>