libstd: Remove unnecessary re-exports under std::std
De-realstd os::args

With the test runner using ::std::os::args(), and std::std::os now being
a re-export of realstd::os, there's no more need for realstd stuff
mucking up rt::args.

Remove the one test of os::args(), as it's not very useful and it won't
work anymore now that rt::args doesn't use realstd.
auto merge of #14430 : kballard/rust/squelch_os_warning, r=alexcrichton

Clean up the re-exports of various modules in `std::std`, and remove the
`realstd` stuff from `std::rt::args`.