added unmapping (invisible) scrollbar gadgets on info sub-screens

Scrollbars and scrollbar up and down buttons were always invisible on
info sub-screens, but could be made visible by blindly clicking on
them if the game window was customized to a small enough height that
the main info screen had a scrollbar and scroll buttons (which were
then not removed again on the info sub-screens, which generally never
have scrollbars and scroll buttons).