rcu/nocb: Remove halfway (de-)offloading handling from rcu_core

RCU core can't be running anymore while in the middle of (de-)offloading
since this sort of transition now only applies to offline CPUs.

The locked callback acceleration handling during the transition can
therefore be removed, along with concurrent batch execution.

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