[cssom] Don't handle CSSViewportRule since it's no longer a thing.

Fixes #8931

Author: emilio

cssom-1/Overview.bs

@@ -1977,8 +1977,6 @@ It must return an integer, as follows:
  <dd>Return 12.
  <dt>If the object is a {{CSSFontFeatureValuesRule}}
  <dd>Return 14.
- <dt>If the object is a {{CSSViewportRule}}
- <dd>Return 15.
  <dt>Otherwise
  <dd>Return 0.
 </dl>