Remove Chapter 27 from reference documentation (SPR-7433)

Chapter 27 covered @Required and RequiredAnnotationBeanPostProcessor but
did not hold together as a chapter unto itself.  The IoC chapter already
makes mention of @Required and RequiredAnnotationBeanPostProcessor,
though not in quite as much detail as Chapter 27 did.  Links have been
updated throughout to reference these sections and Chatper 27 has been
eliminated entirely.

spring-framework-reference/src/beans.xml

@@ -4106,8 +4106,7 @@ org.springframework.scripting.groovy.GroovyMessenger@272961</programlisting>
         <para>Using callback interfaces or annotations in conjunction with a
         custom <interfacename>BeanPostProcessor</interfacename> implementation
         is a common means of extending the Spring IoC container. An example is
-        shown in <xref linkend="metadata-annotations-required" /> which
-        demonstrates the usage of a custom
+        Spring's <classname>RequiredAnnotationBeanPostProcessor</classname> -- a
         <interfacename>BeanPostProcessor</interfacename> implementation that
         ships with the Spring distribution which ensures that JavaBean
         properties on beans that are marked with an (arbitrary) annotation are
@@ -4458,7 +4457,7 @@ dataSource.url=jdbc:mysql:mydb</programlisting>
     annotations is a common means of extending the Spring IoC container. For
     example, Spring 2.0 introduced the possibility of enforcing required
     properties with the <link
-    linkend="metadata-annotations-required">@Required</link> annotation. As of
+    linkend="beans-required-annotation">@Required</link> annotation. As of
     Spring 2.5, it is now possible to follow that same general approach to
     drive Spring's dependency injection. Essentially, the
     <interfacename>@Autowired</interfacename> annotation provides the same