doc: Quick-Quiz answers are now inline

Now that quick-quiz answers are inline, there is no separate section
containing those answers.  This commit therefore removes the dangling
reference from the RCU data-structures design documentation.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>