Replace deprecated API usage for window modal dialogs

Remove the delegate objects that are not used any longer.

Move wxGetAvailableDrives() to Objective-C code file dirdlg.mm to allow
for Cocoa implementation.

include/wx/osx/msgdlg.h

@@ -45,9 +45,6 @@ protected:
     int m_buttonId[4];
     int m_buttonCount;
 
-#if wxOSX_USE_COCOA
-    WX_NSObject m_sheetDelegate;
-#endif
     wxDECLARE_DYNAMIC_CLASS(wxMessageDialog);
 };