locking/barriers: Remove definitions for [smp_]read_barrier_depends()

There are no remaining users of [smp_]read_barrier_depends(), so
remove it from the generic implementation of 'barrier.h'.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>