Remove test for VTK_USE_ANSI_STDLIB and assume availability.  VTK no
longer supports C++ compilers without standard streams.  Define
VTK_USE_ANSI_STDLIB if VTK_LEGACY_REMOVE is OFF so applications that
test for it see the definition.

Change-Id: Ief0c47e8f720754db1916a96f6ed012aa0d8555e