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/statbmp.cpp

@@ -139,7 +139,7 @@ void StatBmpWidgetsPage::RecreateWidget()
         }
         else
         {
-            wxLogMessage("Reading image from file '%s' failed.", filepath.c_str());
+            wxLogMessage("Reading image from file '%s' failed.", filepath);
         }
     }