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: 77a40f97 ("rcu: Remove kfree_rcu() special casing and lazy-callback handling")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>