RCU: Remove prototype for nonexistent function synchronize_idle()

synchronize_idle() sounds like an interesting function, but we don't
actually have it, so don't prototype it.  Introduced in commit
9b06e818985d139fd9e82c28297f7744e1b484e1, in 2005.

Signed-off-by: Josh Triplett <josh@kernel.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/rcupdate.h

@@ -281,7 +281,6 @@ extern void FASTCALL(call_rcu(struct rcu_head *head,
 extern void FASTCALL(call_rcu_bh(struct rcu_head *head,
 				void (*func)(struct rcu_head *head)));
 extern void synchronize_rcu(void);
-void synchronize_idle(void);
 extern void rcu_barrier(void);
 
 #endif /* __KERNEL__ */