Unset widget from sidebar instead of hiding from display

Hiding by filtering widget_display_callback makes is_active_sidebar
return TRUE on  sidewars with only one widget which happens to be
contextually hidden.

Usetting the contextually hidden widget from the sidebar on the front
end restores the desired behavior

Props to Widget Logic.
Merge pull request #4 from emhr/master

Fix for is_active_sidebar & is_dynamic_sidebar issues