overview: Add API to remove search providers

As extensions are now expected to provide a "disable" function,
they need to remove search providers they added. Implement the
removal functionality and add a public removeSearchProvider()
method.

https://bugzilla.gnome.org/show_bug.cgi?id=657548

js/ui/overview.js

@@ -237,6 +237,10 @@ Overview.prototype = {
         this._viewSelector.addSearchProvider(provider);
     },
 
+    removeSearchProvider: function(provider) {
+        this._viewSelector.removeSearchProvider(provider);
+    },
+
     setMessage: function(text, undoCallback, undoLabel) {
         if (this.isDummy)
             return;