rcu: Fix obsolete priority-boosting comment

Tasks are no longer migrated to the root rcu_node, so there is no
longer any need for a boost kthread for the root rcu_node, and there no
longer is such a kthread.  This commit therefore fixes the comment in
rcu_boost_kthread()'s header to reflect this new reality.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>