Remove the separator static line in wxGenericCollapsiblePane

The horizontal static line was added to separate the button from the
child window when visible, but this didn't seem very useful and looked
bad and was inconsistent with the native GTK+ implementation as well as
similar controls commonly used under MSW (wxOSX already disabled the
static line use).

Just remove it to make things simpler and better looking.

Closes https://github.com/wxWidgets/wxWidgets/pull/804

include/wx/generic/collpaneg.h

@@ -13,7 +13,6 @@
 
 // forward declared
 class WXDLLIMPEXP_FWD_CORE wxCollapsibleHeaderCtrl;
-class WXDLLIMPEXP_FWD_CORE wxStaticLine;
 
 #include "wx/containr.h"
 
@@ -80,7 +79,6 @@ protected:
 
     // child controls
     wxCollapsibleHeaderCtrl *m_pButton;
-    wxStaticLine *m_pStaticLine;
     wxWindow *m_pPane;
     wxSizer *m_sz;