Remove unnecessary c_str() calls from the samples

Pass wxStrings directly to wxString::Format("%s") and similar
pseudo-vararg functions, there is no need for c_str() there since
wxWidgets 2.9.

Closes https://github.com/wxWidgets/wxWidgets/pull/1009

samples/widgets/dirpicker.cpp

@@ -247,7 +247,7 @@ void DirPickerWidgetsPage::OnButtonReset(wxCommandEvent& WXUNUSED(event))
 void DirPickerWidgetsPage::OnDirChange(wxFileDirPickerEvent& event)
 {
     wxLogMessage("The directory changed to '%s' ! The current working directory is '%s'",
-                 event.GetPath().c_str(), wxGetCwd().c_str());
+                 event.GetPath(), wxGetCwd());
 }
 
 void DirPickerWidgetsPage::OnCheckBox(wxCommandEvent &event)