rcu: Drop early GP request check from rcu_gp_kthread()

Now that grace-period requests use funnel locking and now that they
set ->gp_flags to RCU_GP_FLAG_INIT even when the RCU grace-period
kthread has not yet started, rcu_gp_kthread() no longer needs to check
need_any_future_gp() at startup time.  This commit therefore removes
this check.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>