rcu: Remove unused srcu_barrier()

The old srcu_barrier() macro is now unused.  This commit removes it so
that it may be used for the SRCU flavor of rcu_barrier(), which will in
turn be needed to allow the upcoming call_srcu() to be used from within
modules.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>