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/radiobox.cpp

@@ -363,7 +363,7 @@ void RadioWidgetsPage::CreateRadio()
     for ( size_t n = 0; n < count; n++ )
     {
         items[n] = wxString::Format("%s %lu",
-                                    labelBtn.c_str(), (unsigned long)n + 1);
+                                    labelBtn, (unsigned long)n + 1);
     }
 
     int flags = m_chkSpecifyRows->GetValue() ? wxRA_SPECIFY_ROWS