docs/memory-barriers.txt: Remove left-over references to "CACHE COHERENCY"

Commit 8ca924ae ("Documentation/barriers: Remove references to
[smp_]read_barrier_depends()") removed the entire section of "CACHE
COHERENCY", without getting rid of its traces.

Remove them.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Andrea Parri <parri.andrea@gmail.com>