Remove unnecessary assignment in wxFileDialog code

"panel" here is an input argument, it's confusing to change it and it's
unnecessary anyhow, so just don't do it.

No real changes.

src/osx/cocoa/filedlg.mm

@@ -650,7 +650,6 @@ void wxFileDialog::ModalFinishedCallback(void* panel, int returnCode)
         NSOpenPanel* oPanel = (NSOpenPanel*)panel;
         if (returnCode == NSModalResponseOK )
         {
-            panel = oPanel;
             result = wxID_OK;
 
             bool isFirst = true;