Checkstyle: cleanup closing tags for modules that have no child tags

[vivekscl]

For tags that have no child tags, there should be no closing tags. Instead, they should end with a slash after the opening tag and have the comment placed above it. For instance,

<module name="FileTabCharacter">
    <!-- Checks that there are no tab characters in the file. -->
  </module>

should be this

<!-- Checks that there are no tab characters in the file. -->
<module name="FileTabCharacter"/>

instead.