rcu: Remove SRCU throttling

The code in srcu_gp_end() inserts a delay every 0x3ff grace periods in
order to prevent SRCU grace-period work from consuming an entire CPU
when there is a long sequence of expedited SRCU grace-period requests.
However, all of SRCU's grace-period work is carried out in workqueues,
which are in turn within kthreads, which are automatically throttled as
needed by the scheduler.  In particular, if there is plenty of idle time,
there is no point in throttling.

This commit therefore removes the expedited SRCU grace-period throttling.

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