rcu: Remove references to rcu_migrate_callbacks() from diagrams

This function is gone since:

	53b46303

 (rcu: Remove rsp parameter from rcu_boot_init_percpu_data() and friends)

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>