These tests originally existed to check the containers in C++11 mode,
when the default was C++98 mode. Now that the default is C++14 (and we
run most tests for all modes) it serves no purpose to have two copies of
the tests when neither is explicitly using -std=gnu++98 anyway.
5_c++0x.cc: Remove redundant test that is functionally identical to
the 5.cc test.