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>