remove WXWIN_COMPATIBILITY_2_6, add WXWIN_COMPATIBILITY_3_0

closes #15792

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75532 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/txtstrm.h

@@ -75,10 +75,6 @@ public:
 
     wxTextInputStream& operator>>( __wxTextInputManip func) { return func(*this); }
 
-#if WXWIN_COMPATIBILITY_2_6
-    wxDEPRECATED( wxString ReadString() );  // use ReadLine or ReadWord instead
-#endif // WXWIN_COMPATIBILITY_2_6
-
 protected:
     wxInputStream &m_input;
     wxString m_separators;