doc: Remove obsolete rcutree.rcu_idle_lazy_gp_delay boot parameter

This commit removes documentation for the rcutree.rcu_idle_lazy_gp_delay
kernel boot parameter given that this parameter no longer exists.

Fixes: 77a40f9 ("rcu: Remove kfree_rcu() special casing and lazy-callback handling")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4179,12 +4179,6 @@
 			Set wakeup interval for idle CPUs that have
 			RCU callbacks (RCU_FAST_NO_HZ=y).
 
-	rcutree.rcu_idle_lazy_gp_delay= [KNL]
-			Set wakeup interval for idle CPUs that have
-			only "lazy" RCU callbacks (RCU_FAST_NO_HZ=y).
-			Lazy RCU callbacks are those which RCU can
-			prove do nothing more than free memory.
-
 	rcutree.rcu_kick_kthreads= [KNL]
 			Cause the grace-period kthread to get an extra
 			wake_up() if it sleeps three times longer than