Don't include warning headers from catch_xmlwriter.hpp

This prevents Catch from disabling `Wpadded` for Clang inside test files
(files that do not define either `CATCH_CONFIG_MAIN` or
`CATCH_CONFIG_RUNNER`).

catch_suppress_warnings.h and catch_reenable_warnings.h should be
included only once*, so that the stitching script includes them as the
first and last header respectively, since it only includes each header
once. This caused a bug, where the first one was included properly, but
the second one was included prematurely, from catch_xmlwriter.hpp, and
thus was guarded by `CATCH_IMPL`.

* At least until the stitching script is changed to accomodate common
warning disabling header.

Fixes #871

include/internal/catch_xmlwriter.hpp

@@ -10,7 +10,6 @@
 
 #include "catch_stream.h"
 #include "catch_compiler_capabilities.h"
-#include "catch_suppress_warnings.h"
 
 #include <sstream>
 #include <string>
@@ -241,6 +240,5 @@ namespace Catch {
     };
 
 }
-#include "catch_reenable_warnings.h"
 
 #endif // TWOBLUECUBES_CATCH_XMLWRITER_HPP_INCLUDED