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/spinbtn.cpp

@@ -587,7 +587,7 @@ void SpinBtnWidgetsPage::OnSpinCtrlDouble(wxSpinDoubleEvent& event)
 void SpinBtnWidgetsPage::OnSpinText(wxCommandEvent& event)
 {
     wxLogMessage("Text changed in spin control, now \"%s\"",
-                 event.GetString().c_str());
+                 event.GetString());
 }
 
 void SpinBtnWidgetsPage::OnSpinTextEnter(wxCommandEvent& event)