Cleanup: remove outdated comments on RCU requirements

Cleanup: remove outdated comments on RCU requirements

The ht-cleanup thread was introduced to work-around the limitation that
the hash table destroy could not be called from a RCU read-side critical
section because of requirements from the rculfhash API, some comments
stating that destruction code should not be called from RCU read-side
critical sections were left in place when they should in fact have been
removed.

This ht-cleanup thread work around was introduced by the following
commit:
  commit 0b2dc8df2a6d7b3341a72a04767dd6328907c97c
  Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  Date:   Fri Jun 14 07:44:52 2013 -0400

    Fix: hash table cleanup call_rcu deadlock

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ifdaa63e6146b51fdb251e75795e382b5f390f7be