nss-myhostname: do not use _cleanup_

mss-myhostname wasn't working because of underlinking. Instead of
fixing the underlinking, just remove the use of _cleanup_ macros.

It is impolite to use our utility functions in modules designed to be
loaded by others. So cleanup macros which (at some point) call assert
which calls log_assert_failed, should not be used. Revert this part of
commit d73c3269c.