_exit.2: Open stdio frames are not flushed, temporary files are deleted

_exit.2: Open stdio frames are not flushed, temporary files are deleted

Many years ago, text was added to the page saying that it is
implementation-dependent whether stdio streams are flushed and
whether temporary are removed. In part, this change appears to
be because POSIX.1-2001 added text related to this point.
However, that seems to have been an error in POSIX, and the
text was subsequently removed for POSIX.1-2008. See
https://collaboration.opengroup.org/austin/interps/documents/9984/AI-085.txt
Austin Group Interpretation reference 1003.1-2001 #085

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>