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/clrpicker.cpp

@@ -218,7 +218,7 @@ void ColourPickerWidgetsPage::OnButtonReset(wxCommandEvent& WXUNUSED(event))
 void ColourPickerWidgetsPage::OnColourChange(wxColourPickerEvent& event)
 {
     wxLogMessage("The colour changed to '%s' !",
-                 event.GetColour().GetAsString(wxC2S_CSS_SYNTAX).c_str());
+                 event.GetColour().GetAsString(wxC2S_CSS_SYNTAX));
 }
 
 void ColourPickerWidgetsPage::OnCheckBox(wxCommandEvent &event)