idle: Remove stale RCU comment

The call of rcu_idle_enter() from within cpuidle_idle_call() was
removed in commit 1098582a ("sched,idle,rcu: Push rcu_idle deeper
into the idle path") which makes the comment out of place.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/5b936388-47df-4050-9229-6617a6c2bba5@arm.com