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/power/power.cpp

@@ -179,8 +179,8 @@ private:
 
         SetStatusText(wxString::Format(
                         "System is on %s power, battery state is %s",
-                        powerStr.c_str(),
-                        batteryStr.c_str()));
+                        powerStr,
+                        batteryStr));
     }
 
     void OnStartTaskClicked( wxCommandEvent& WXUNUSED(event) )