search: Don't expand search when clicking section headers

Currently section headers in the search view are reactive and run
the corresponding provider's expandSearch() function when clicked,
which should launch an external program displaying all search
results for the section. Unfortunately it is only implemented for
the "Settings" provider, and does not work properly (as it ignores
the search and just launches System Settings).
Also current mockups deemphasize the section header, making the
feature pretty much indiscoverable (except by accident when
interfering with swipe-scrolling).
In conclusion, the feature does not make much sense in its current
form, so remove it.

https://bugzilla.gnome.org/show_bug.cgi?id=643632

js/ui/search.js

@@ -209,17 +209,6 @@ SearchProvider.prototype = {
      */
     activateResult: function(id) {
         throw new Error('Not implemented');
-    },
-
-    /**
-     * expandSearch:
-     *
-     * Called when the user clicks on the header for this
-     * search section.  Should typically launch an external program
-     * displaying search results for that item type.
-     */
-    expandSearch: function(terms) {
-        throw new Error('Not implemented');
     }
 };
 Signals.addSignalMethods(SearchProvider.prototype);