[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.