Remove obsolete symbols and platforms from the documentation

Don't mention pre-XP Windows versions and other obsolete platforms such as
OS/2 and the symbols which were only useful for them.

Closes https://github.com/wxWidgets/wxWidgets/pull/137

interface/wx/notebook.h

@@ -23,7 +23,6 @@ enum
 #define wxNB_FIXEDWIDTH       0x0100
 #define wxNB_MULTILINE        0x0200
 #define wxNB_NOPAGETHEME      0x0400
-#define wxNB_FLAT             0x0800
 
 wxEventType wxEVT_NOTEBOOK_PAGE_CHANGED;
 wxEventType wxEVT_NOTEBOOK_PAGE_CHANGING;
@@ -58,8 +57,6 @@ wxEventType wxEVT_NOTEBOOK_PAGE_CHANGING;
     @style{wxNB_NOPAGETHEME}
            (Windows only) Display a solid colour on notebook pages, and not a
            gradient, which can reduce performance.
-    @style{wxNB_FLAT}
-           (Windows CE only) Show tabs in a flat style.
     @endStyleTable
 
     The styles wxNB_LEFT, RIGHT and BOTTOM are not supported under