rcu: Remove callback acceleration from grace-period initialization

rcu: Remove callback acceleration from grace-period initialization

Before grace-period initialization was moved to a kthread, the CPU
invoking this code would have at least one callback that needed
a grace period, often a newly registered callback.  However, moving
grace-period initialization means that the CPU with the callback
that was requesting a grace period is not necessarily the CPU that
is initializing the grace period, so this acceleration is less
valuable.  Because it also adds to the complexity of reasoning about
correctness, this commit removes it.

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