vector/screen.css: Remove some unnecessary rules

* There is no label inside div#simpleSearch.
* Inside div#simpleSearch, input and input#searchInput are the
  same in practice (there is a <input type=submit>, but it is hidden).

Change-Id: I1d9657a5ab608d1a54314dd7c96144af96c53d59
Merge "vector/screen.css: Remove some unnecessary rules"