rcu/nocb: Remove obsolete nocb_q_count and nocb_q_count_lazy fields

This commit removes the obsolete nocb_q_count and nocb_q_count_lazy
fields, also removing rcu_get_n_cbs_nocb_cpu(), adjusting
rcu_get_n_cbs_cpu(), and making rcutree_migrate_callbacks() once again
disable the ->cblist fields of offline CPUs.

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