tools/memory-model: Remove smp_read_barrier_depends() from informal doc

smp_read_barrier_depends() has gone the way of mmiowb() and so many
esoteric memory barriers before it. Drop the two mentions of this
deceased barrier from the LKMM informal explanation document.
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

tools/memory-model/Documentation/explanation.txt

@@ -1122,12 +1122,10 @@ maintain at least the appearance of FIFO order.
 In practice, this difficulty is solved by inserting a special fence
 between P1's two loads when the kernel is compiled for the Alpha
 architecture.  In fact, as of version 4.15, the kernel automatically
-adds this fence (called smp_read_barrier_depends() and defined as
-nothing at all on non-Alpha builds) after every READ_ONCE() and atomic
-load.  The effect of the fence is to cause the CPU not to execute any
-po-later instructions until after the local cache has finished
-processing all the stores it has already received.  Thus, if the code
-was changed to:
+adds this fence after every READ_ONCE() and atomic load on Alpha.  The
+effect of the fence is to cause the CPU not to execute any po-later
+instructions until after the local cache has finished processing all
+the stores it has already received.  Thus, if the code was changed to:
 
 	P1()
 	{
@@ -1146,14 +1144,14 @@ READ_ONCE() or another synchronization primitive rather than accessed
 directly.
 
 The LKMM requires that smp_rmb(), acquire fences, and strong fences
-share this property with smp_read_barrier_depends(): They do not allow
-the CPU to execute any po-later instructions (or po-later loads in the
-case of smp_rmb()) until all outstanding stores have been processed by
-the local cache.  In the case of a strong fence, the CPU first has to
-wait for all of its po-earlier stores to propagate to every other CPU
-in the system; then it has to wait for the local cache to process all
-the stores received as of that time -- not just the stores received
-when the strong fence began.
+share this property: They do not allow the CPU to execute any po-later
+instructions (or po-later loads in the case of smp_rmb()) until all
+outstanding stores have been processed by the local cache.  In the
+case of a strong fence, the CPU first has to wait for all of its
+po-earlier stores to propagate to every other CPU in the system; then
+it has to wait for the local cache to process all the stores received
+as of that time -- not just the stores received when the strong fence
+began.
 
 And of course, none of this matters for any architecture other than
 Alpha.