gtk/textview: Do not forcibly show text handles

gtk/textview: Do not forcibly show text handles

The gtk_text_view_set_handle_position() function called some lines above
takes care of handle visibility already, also accounting for other
conditions (e.g. whether the handle points to contents onscreen).
Forcibly showing handles here misbehaves if the handle should stay hidden,
and somewhat expensively as it involves creating and throwing a native
surface every time.