Remove useless wxFont::ActivatePrivateFonts()

Just "activate" the font immediately when adding it using
AddPrivateFont(), nothing seems to be gained from having two functions
and it just makes things more complicated both when implementing and
when using the API.

src/common/fontcmn.cpp

@@ -1148,9 +1148,4 @@ bool wxFontBase::AddPrivateFont(const wxString& WXUNUSED(filename))
     return false;
 }
 
-bool wxFontBase::ActivatePrivateFonts()
-{
-    return false;
-}
-
 #endif // !wxHAS_PRIVATE_FONTS