libstdc++: Simplify numeric_limits<__max_size_type>

libstdc++: Simplify numeric_limits<__max_size_type>

If __int128 is supported then __int_traits<__int128> is guaranteed to be
specialized, so we can remove the preprocessor condition inside the
std::numeric_traits<__detail::__max_size_type> specialization. Simply
using __int_traits<_Sp::__rep> gives the right answer.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/max_size_type.h (numeric_limits<__max_size_type>):
	Use __int_traits unconditionally.