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

src/common/txtstrm.cpp

@@ -197,15 +197,6 @@ double wxTextInputStream::ReadDouble()
     return wxStrtod(word.c_str(), 0);
 }
 
-#if WXWIN_COMPATIBILITY_2_6
-
-wxString wxTextInputStream::ReadString()
-{
-    return ReadLine();
-}
-
-#endif // WXWIN_COMPATIBILITY_2_6
-
 wxString wxTextInputStream::ReadLine()
 {
     wxString line;