rcu: Eliminate rcu_state structure's ->call field

The rcu_state structure's ->call field references the corresponding RCU
flavor's call_rcu() function.  However, now that there is only ever one
rcu_state structure in a given build of the Linux kernel, and that flavor
uses plain old call_rcu(), there is not a lot of point in continuing to
have the ->call field.  This commit therefore removes it.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.h

@@ -313,7 +313,6 @@ struct rcu_state {
 						/* Hierarchy levels (+1 to */
 						/*  shut bogus gcc warning) */
 	struct rcu_data __percpu *rda;		/* pointer of percu rcu_data. */
-	call_rcu_func_t call;			/* call_rcu() flavor. */
 	int ncpus;				/* # CPUs seen so far. */
 
 	/* The following fields are guarded by the root rcu_node's lock. */