rcu: Remove references to old grace-period-wait primitives

The rcu_barrier_sched(), synchronize_sched(), and synchronize_rcu_bh()
RCU API members have been gone for many years.  This commit therefore
removes non-historical instances of them.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>