doc: rcu: Remove obsolete checklist item about synchronize_rcu usage

Since the RCU mechanisms have been consolidated, the checklist item
warning that synchronize_rcu() waits only for RCU readers is obsolete.
This commit therefore removes this checklist item.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>