Get rid of platform-specific wxPanel classes

There is no need to have them as they were identical for all platforms.

This basically reverts 5b87bd6c0b (Split wxPanel in wxPanelBase and
platform-specific files., 2011-03-20) as, 10+ years later, it turns out
that we're not going to have more MSW-specific parts, but none of them.

No real changes.

build/files

@@ -1072,7 +1072,6 @@ GUI_CMN_HDR =
     wx/generic/notebook.h
     wx/generic/notifmsg.h
     wx/generic/numdlgg.h
-    wx/generic/panelg.h
     wx/generic/printps.h
     wx/generic/prntdlgg.h
     wx/generic/progdlgg.h
@@ -1808,7 +1807,6 @@ MSW_HDR =
     wx/msw/ownerdrawnbutton.h
     wx/msw/ownerdrw.h
     wx/msw/palette.h
-    wx/msw/panel.h
     wx/msw/pen.h
     wx/msw/printdlg.h
     wx/msw/printwin.h
@@ -2347,7 +2345,6 @@ UNIV_HDR =
     wx/univ/menu.h
     wx/univ/menuitem.h
     wx/univ/notebook.h
-    wx/univ/panel.h
     wx/univ/radiobox.h
     wx/univ/radiobut.h
     wx/univ/renderer.h