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/toolbar/toolbar.cpp

@@ -853,7 +853,7 @@ void MyFrame::OnToolRightClick(wxCommandEvent& event)
 
 void MyFrame::OnCombo(wxCommandEvent& event)
 {
-    wxLogStatus("Combobox string '%s' selected", event.GetString().c_str());
+    wxLogStatus("Combobox string '%s' selected", event.GetString());
 }
 
 void MyFrame::DoEnablePrint()