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

Commit 8ca924a ("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>

Author: akiyks

Documentation/memory-barriers.txt

@@ -88,7 +88,6 @@ CONTENTS
 
  (*) The effects of the cpu cache.
 
-     - Cache coherency.
      - Cache coherency vs DMA.
      - Cache coherency vs MMIO.
 
@@ -677,8 +676,6 @@ include/linux/rcupdate.h.  This permits the current target of an RCU'd
 pointer to be replaced with a new modified target, without the replacement
 target appearing to be incompletely initialised.
 
-See also the subsection on "Cache Coherency" for a more thorough example.
-
 
 CONTROL DEPENDENCIES
 --------------------