Drupal Common's 3 How To Edit The Commons Browsing Widget Tabs?
If you specifically need to remove the Wiki's Tab instead of turning off Commons Wiki module, none of the functions to override has been set up as a hook, as far as I can understand.

So even if the following method is not the best approach, you could make a very quick patch to commons_bw.module :

Into function commons_bw_get_tab_definitions($widget_type = 'group')

add unset($tabs['commons_wikis']); before return $tabs;

Of course you'll have to remember this whenever you update your Commons distrib...

See also this great post on how to add your own content type to the Commons browsing widget. This will give you a complete overview of what you can do