[cssom-1] Simplify setting MediaList.mediaText

[cdoublev]

This PR removes step 2 for setting MediaList.mediaText with empty string since step 3 (parsing) produces the same outcome:

3. Append all the media queries as a result of parsing the given value to the collection of media queries.

Parsing links to parse a media query list which redirects to <media-query-list>:

To parse a <media-query-list> production, parse a comma-separated list of component values, then parse each entry in the returned list as a <media-query>.

Note: This definition of <media-query-list> parsing intentionally accepts an empty list.

Note that the entire procedure could be replaced with "replace the collection of media queries with the result from parsing the given value", since this collection is not exposed nor shared with another interface.