Drop unused CXX cstdio check

The `KWSYS_CXX_HAS_CSTDIO` check has been unused since commit 363bf5ab
(Configure: Drop Large File Support macro definitions, 2019-02-22).

kwsysPlatformTestsCXX.cxx

 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing#kwsys for details.  */
-#ifdef TEST_KWSYS_CXX_HAS_CSTDIO
-#  include <cstdio>
-int main()
-{
-  return 0;
-}
-#endif
 
 #ifdef TEST_KWSYS_CXX_STAT_HAS_ST_MTIM
 #  include <sys/types.h>